What seems to be an account of Isherwood's first full day in Berlin is headed "Saturday getting tight," and later come "Intervening period of Margaret and sex expectation," "Meeting Gerhart and John's expected suicide," "The Dutchman and the Tour." [...] Auden told his brother that he was "writing another play." [...] This play was The Fronny -- again named after Francis Turville-Petre, who may well have been a topic of conversation between Auden and Isherwood during that June visit.
"Gerhart" also comes up in Auden's lines about "absence of fear in Gerhart Meyer / From the sea, the truly strong man." (IIRC they were lovers at some point.) Fronny -- i.e., FTP -- was an interesting character by all accounts, and excellently named. Here is some more about an early draft of the Fronny, called The Enemies of a Bishop aka Where is Fronny?:
One of the characters is a pederast, another is based on a character in the film Der Student von Prag, and the Bishop himself is based on Homer Lane. Auden introduced one of his favorite lead mines, and Isherwood contributed characters who owed much to the fantasy world he and [Edward] Upward had created together.The Isherwood-Upward fantasy world was Mortmere, which (according to Lions and Shadows) was once provisionally named Stoat Grange. (One supposes The Wind in the Willows was a large influence on Isherwood, though I don't think he mentions having read it.)
Unrelated: the string "Aer Lingus" has an inordinate number of anagrams: e.g., "e.g., urinals," "seal ruing," "earl suing," "glue rains," "sugar line," "realign us," "alien rugs," "silage urn," "sea ruling," "ursine gal," and "a slier gnu."