I came accros this lens some time ago .
Strangely enough these are hard to find in a m42 mount .
Just like most others this one had a exakta mount .
Here's a little report on how i changed the mount to a m42 mount .
The whole process can be reversed .
Results can be seen in the link below .
https://pbase.com/our_shots/zeis_jena_flektogon_25mm_f4