Old Zenit SLRs like the Zenit 3M have another M39 thread mount. It is a little bit different to the Leica M39.
If you screw a M39 Helios 44 2/58mm on a Fed or a Zorki, you won't get a sharp picture. It's just impossible to focalize.
Even so you can use this effect for special pictures, like this one with a FED 4 and a Helios 44 2/58mm:
|"the creature merges in his surroundings", germany, 2006|
Here an example shot with this camera-lens combination:
|"light hole", brasil, 2007|
I have these ideas from Alfred Klomp's brilliant homepage.