Some of the available lists of twentieth-century papal claimants include a Julius Tischler, who asserted that he was Pope Peter II. Most lists do not include any details on him, while some note that he was born in 1908. To my knowledge, the only researcher who provides some information about the case is Joachim Bouflet, who briefly mentioned Tischler in his well-researched Faussaires de Dieu. Though he does not refer to any primary sources, Bouflet underlined that Julius Tischler was not a real name, but a pseudonym, and that he was not strictly a papal claimant, but a claimant to a future papacy; he would become the last pope.
The main source of Julius Tischler’s claims is his 336-page-book published in 1972: Der Handwerksgeselle: Der Vierte Seher von Fatima [The Journeyman: The Fourth Seer of Fatima.] It is a peculiar book. On the one hand, it is a very detailed autobiography about the author’s first fifteen years in life, following a strictly chronological outline. On the other, the author included many accounts of amazing spiritual experiences, which he seamlessly intertwined with the much more down-to-earth autobiographical narrative. Among other things, the author claimed that he was mystically present at Fatima when the Virgin appeared in 1917. In short, he was the fourth seer of Fatima. Moreover, he was the only of the children who received the second, most important part of the Virgin’s message in 1923. The author’s real name was Franz Engelhardt (or Ferenc Egerszegi) who was a Roman Catholic priest of Hungarian origin who was a parish priest in the diocese of Trier.
A preliminary version of my text on Tischler/Engelhardt is found here