Adolf Hitler enjoyed special treatment while jailed in 1924 and was allowed hundreds of visitors – sometimes unsupervised – including some 30 to 40 celebrants of his 35th birthday. The details have emerged from documents written by officials at the prison near Munich where he was held.

The 500 papers from the Landsberg prison were recently found by a Nuremberg man among the possessions of his late father, who purchased them at a flea market in the 1970s according to Werner Behringer, whose auction house in the Bavarian city of Fuerth will offer them for sale next month.

Mr Behringer said they were packed among a bundle of books on the First World War the man had bought, and his 55-year-old son, who has requested anonymity, never knew of their existence. "His father probably didn't know what he had there," Mr Behringer told Associated Press. Robert Bierschneider, an archivist with the Bavarian State Archives in Munich, said he had examined images of the documents Mr Behringer sent to him and they had stamps and notations matching others from the prison.

"The documents appear genuine, but to do a real examination we need the originals in our hands," he said.

The documents are to be auctioned on 2 July, with a starting price of €25,000 (£20,000). Though only one is signed by Hitler, and much of the information is otherwise available, they do provide an intriguing window into his early days as Nazi leader.

Hitler was imprisoned in Landsberg after his abortive bid to seize power in the notorious "beer hall putsch" in Munich. A decade later, in 1933, the Nazis came to power through elections.

Sentenced to five years in prison, Hitler was granted early release and ended up serving about nine months.

His right-wing politics and German nationalism won him friends among the German establishment, including the First World War hero Erich Ludendorff. He came to visit Hitler several times in jail and the Prussian general was allowed to see the former Austrian corporal unsupervised for as long as he wanted, the documents show.

They include some 300 to 400 original cards listing Hitler's other visitors, including the 30 to 40 allowed in to celebrate his birthday on 20 April 1924 – 19 days into his sentence. "His time in prison was more like a holiday," Mr Behringer said.

Otto Leybold, the prison director, gushed about Hitler in a memo about the inmates on 18 September 1924, saying he was always "sensible, modest, humble and polite to everyone – especially to the officers of the facility."

Hitler spent much of his time in prison writing his manifesto Mein Kampf, detailing his ideology and ambitions, but the documents also show he had time for more prosaic thoughts.

In a typed copy that prison authorities made of a letter Hitler wrote to a Munich car dealer, the future dictator is having a hard time deciding whether to purchase a newer model Benz 11/40 or the older 16/50 because he had concerns that the higher RPMs of the motor in the former might mean that it would have more mechanical problems. "I can't get a new car every two or three years," he wrote.