Hello Jim,
Jose (Hernandez-Sosa) here. Haven't read about you since the old Walther PP & PPK forum. Glad to know you're fine.
My guess is as good as anybody elses. However, I do agree with you that if a choice has to be made, the shortened 9 Luger makes more sense than the overstretched 9mm K. If they were looking for more umpphh in a compact package, it seems the logical thing to do. Most of us have seen the unacknowledged diagram that purports to detail the specs of the 1936(?) Ultra cartridge. See,
http://makarov.com/graphics/9mmcases.jpg
Problem is, no one has ever acknowledged fathering it. It seems to be one of those things that suddenly appear out-of-the-blue and then remains as an icon waiting for validation. One of two possibilities exist with regard to the 1935-1936 specs. Either the Nazis burned all its relevant documentation, which they were fond of doing at the end of WW2( just ask Herrn Marschall) or the Soviets absconded it from the world(perhaps thinking that it was the elusive 'Vengeance Weapon Drei'). I have the feeling that it was turned to ashes, but then again my guess is as good as anyones.
This is how I feel about it. The specs of the 1935-1936 9x18 Ultra cartridge are the same as the post-war 9x18 Ultra cartridge. Only because there's so much that can be done for a midway round between the 9x17 and the 9x19. And Geco and Walther were the ones who first came up with it, on both counts. Then apparently, Benelli, Sig, Tanfoglio and Mauser(Renato Gamba) in that order, all late 70s to early 80s. Therefore, the post-war 9x18 Ultra is the same cartridge as the 9x18 Police. Geco and Hirtenberger call it Ultra. Fiocchi calls it Police. Semantics. Geco and Hirtenberger call the bullet a flat nose. Fiocchi calls it a truncated cone. Semantics again. Fiocchi probably used the term merely as a selling point. A marketing strategy that takes advantage of the fact that it was supposed to be the 'wunder' police cartridge when it first came out post-war. Until groups like Baader-Meinhof entered the scene.
And no, I do think the Soviets did not copy from the Ultra to come up with the Makarov round. Then again, there's just so much that can be done with a midway round between 9x17 and 9x19. Further, no one has really explained the why of a .363 bullet in the Mak as opposed to a .355 in the 9 Luger family. Looks to me that they came up with it all on their own. A testament to Mr. Semin's ingenuity. Just don't know of any other round using a .363 bullet.