I wrote a whole novel about the Jupiter 12 and my quest to adapt it to my new Sigma fp. Tl;dr? The Jupiter 12 was a major driver behind my purchase of the fp, in fact — how pathetic! — but the regular m39 version will not fit.

Turns out my theorizing was correct: The Contax RF version of the Jupiter-12 fits!

My setup is:

1. Jupiter-12 Kiev (Contax/Nikon RF) mount +
2. Yeenon Contax/Nikon RF to Leica M adapter +
3. Haoge Leica M with close focusing helicoid to L mount adapter +
4. Sigma fp

The lens has quite a lot of wiggle/play on the Yeenon adapter, but it firms up as it focuses to infinity. The other lens I have in this mount mounts firmly on the adapter with no play, so that's an issue. I've written Yeenon to ask if they have any ideas.

The fact that it gets nice and firm when focused to infinity is a little concerning to me but I've inspected the interior of the camera and found zero signs of contact or damage.

I haven't had a change to take the lens outside yet, but here's a test shot:

I'm really pleased with the rendering of this lens, as always… in-focus elements are clearly in focus, with the famous 3D pop, but they're also delicate and soft and friendly, somehow, too. But not soft in a "this lens is not sharp" way. It's a sharp softness.