Sonnar 135mm f4-what mount?

[sevo]

The front element is recessed by ~9mm with respect to the outer end of the filter ring.

The lens was part of a deal with an enlarger, so it might have been used at some point in time as an enlarging lens, although I fail to see the point of doing that.

M39 is a popular enlarger thread, and the lens would cover up to 9x12 - not up to particularly high performance standards, but the shortage of LF gear accessible to self-employed photographers in the GDR created even stranger modifications.

 

[Dwig]

I have just measured the flange distance that way and it seems to be 35mm or so, but I'm not sure how precise I was.

EDIT: remeasured it and it is more like 42mm

There were several SLRs that used an M39 thread mount with the same 45.46mm register as the later and more common M42. These included the original Asahi reflex and the Zenit 3M

The Pratiflex of 1939 used an M40 mount with a 44mm register and the Paxette used an M39 mount with the same 44mm register. Leitz' original Viso mount is not easily confused with since while used the 39mm LTM thread its register is a very large 91.3mm.