'He was most informative.'

'Are you with the Diplomatic Corps?' This time there was a smile and a twitch of the nose. 'Or is London Central sending us a more subtle type of man these days?' Staiger was a decade younger than me and yet he could get away with such a remark without inciting anger or resentment. Baron Staiger of Vienna, and Herr Hoffman of Salzburg, and God knows what in the other places he went, was provided with more than his full share of that Viennese Zauber that the rest of the world calls schmaltz.

He said, 'Andras has had a disappointing evening, I'm afraid. He has spent ten years trying to get his string quartet performed. Tonight it was. His loyal friends went but there were not enough of us to fill the hall.' He sipped his drink. 'Worse still, I think Andras realized that his composition wasn't really very good.'

'Poor Andras,' I said.

'His parents own the Scolik Konditorei,' said Staiger ironically. 'Know it? Each afternoon old ladies stand in lines to devour that superb Scolik poppy-seed strudel with a big dollop of Schlagobers. It is like owning a gold mine. The strudel will help him survive his crisis of confidence.'

'Is that what he's having?'

'Strudel?' he asked mockingly. 'No, you mean a crisis of confidence. Tomorrow he will face the music critics,' said Staiger. 'And Vienna breeds a savage race of critics.'

'Karl!' said a small sharp-featured woman who soon made it evident from her manner that she was Staiger's wife. Ignoring me she said, 'Anna-Klara has arrived, Karl.' She touched his arm. I wondered if she knew about her husband's other lives. Perhaps she thought I was a part of them.

Staiger smiled in a satisfied way. 'She has? Kolossal!' I was later to discover that he considered this lady's visit a social coup of some magnitude. He looked round to make sure that there was no aspect of the room that would disgrace him in the eyes of this renowned visitor, and found only me. For a moment I thought he would hide me in a cupboard, but he swallowed, looked at his wife apologetically and – as if explaining his predicament – said, 'When the guests have gone home, I have some work to do with Herr Doktor Samson.' He smoothed his thinning hair as if checking that it was in place.

The wife looked at me and nodded grimly. She knew I wasn't really a Doktor, a real Doktor would have been called 'Baron' and a real Baron 'Prince'. That's how things worked in Austria. I smiled but she didn't respond. She was a dutiful Austrian wife who let her husband make decisions about his work. But she didn't have to like his down-at-heel work-mates. 'Here comes Anna-Klara,' she said.

The arrival of the guest of honour was what they had all been waiting for. This soprano had been performing at the opera that night, and when she came into the room it was an entrance befitting the reverence that this assembled audience afforded her. She swept in with a flourish of the long flowing skirt. Her yellow hair was piled high and glittering with jewels. Her make-up was slightly overdone, but that was de rigueur for someone who'd hurried from the opera stage.

Her fellow guests greeted her with a concerted murmur of awe and devotion. With the Staigers at her side, the gnädige Frau went from one to another of them like a general inspecting a guard of honour. Here, bowing low, was a Doktor Doktor and a Frau Doktor, his wife; the bureaucrat's wife – Frau Kommerzialrat – gave a sort of a curtsy; the Hofrat – court adviser for a Habsburg Emperor long since dead and gone – kissed her hand. Anna-Klara had gracious words for all of them, and special compliments for Andras Scolik and the string quartet performance she'd missed. Scolik brightened. Anna-Klara had praised him. And, after all, there was always the strudel.

It was a bravura performance, and with impeccable instinct Anna-Klara stayed for only one glass of champagne before departing again. Once she had gone the party broke up quickly.

It was midnight when I sat down with Karl Staiger in his office at the back of the shop. All the church clocks in Vienna were proclaiming the witching hour. The room smelled of varnish, and Staiger opened the window a fraction despite the bitter cold night outside. Then he moved a lot of unopened mail from where it was leaning against an antique carriage clock and compared the time with that on his wristwatch. It was a beautiful clock, its face decorated with dancing ladies. The movement ticked happily inside the glass-sided case. He nodded proudly at me as a father might smile to see his child play the piano for guests. Satisfied, he moved more books and papers to clear a space on his desk where a green-shaded lamp made a perfect circle of light upon a pink blotter.

'What happened?' said Staiger.

'I haven't got it,' I said. I had no intention of talking to him about the death of Johnson, or mentioning Thurkettle and his possible role in the murder.

'Haven't got what?' He had his arms loaded with books.

From my jacket pocket I produced my wallet and I laid the coloured photo of the cover exactly in the centre of the pool of light. This,' I said, smoothing it out. 'I haven't got this.'

He put the books on to a cupboard and looked down at the photo. Then, without speaking, he took the bundle of unopened mail propped against the clock. Going quickly through it, he chose a packet that bore the large and impressive-looking labels of a courier company. It was a small padded bag secured with metal staples. He tore it open with an effortless twist and shook the contents from it.

On to the table slid a blue envelope with Paraguay stamps and Zeppelin marks: the same cover as that depicted in the colour photo upon which it fell.

'But I've got it,' said Staiger with a satisfied smile.

'What's the story?' I picked up the cover that had caused so much trouble and probably brought about the amiable Johnson's death. I turned it in my hands. It seemed such a useless piece of paper to be sold for such a high price.

'I only know what I can read between the lines,' he said. 'But I think the Americans sent someone to buy it over your head. I had to get on to one of the biggest dealers in Vienna – an old friend – and ask him to get it at all costs.'

'He must have phoned his bids.'

'There was no time for anyone to get to Salzburg.'

'The room bidder was chiselled, the auction was rigged. At least, that bid was.'

'These things happen,' said Staiger. 'I had no idea the Americans would try to intervene or I would have given you more cash. But it turned out all right. I was told to get it; I got it.' He picked up the cover and held it against the light.

'Is there something inside?'

'Usually there is some stiffening to protect such covers, a piece of card, sometimes one that advertises some long-forgotten stamp dealer.' But while saying this he took from the drawer of his desk a beautiful ivory letter opener and tapped it against his hand. 'You know that the best items in the sale were from a private collection put together in the nineteen thirties by a famous Hungarian airpost dealer named Zoltan Szarek. He was the author of the 1935 Szarek Airpost Manual, long out of print. Now that the Szarek collection is broken up it is the end of one of the world's greatest.' He turned the letter opener round. One end of it concealed a tiny penknife blade. He opened the blade and to my surprise cut open the precious Paraguay envelope.

Having seen the sort of passion that these philatelic objects aroused in men like Staiger I was amazed at this vandalism. But there was a surprise to come, for inside the blue envelope there were two passport-sized photos. The photos were obviously recent ones. The people had grown older since the last time I'd seen them and the photos were dull and lacking in true blacks because they were printed on that sort of grey-toned photo-paper that is used in countries that can't afford much silver. He placed them on the blotter in front of me. 'Anyone you know?'