Pentacon 300mm f/4 won’t mount on Kiev 6C – wrong mount installed?

[Medium_KV6C]

I’m hoping someone with experience of older Soviet / East German gear can help me figure this out.

I’ve got a Kiev 6C - which has a Pentacon Six mount and a few years ago picked up a Pentacon 300mm f/4 (Orestegor). The lens looks to have an interchangeable rear mount system.

Here’s the issue:

• The lens has bayonet lugs, so it’s not just a flat plate or M42 thread
• But when I try to mount it, it feels slightly too small
• It won’t properly lock into the camera, other lenses on the camera lock in just fine
• The locking pin on the body doesn’t align or engage properly
• There are 3 small screws holes on the rear mount and one larger one, but that hole doesn’t seem to match the Kiev’s locking pin
• The 3 larger screws on the base are also soft metal and with age are nearly entirely flat meaning that they are also hard to remove so close to buggering it.

So I’m trying to work out:

1. Does this sound like the lens has the wrong mount installed (e.g. Exakta or something else)?
2. Were there multiple interchangeable mount types for the Orestegor 300mm that look similar to Pentacon Six but aren’t?
3. Do I need to source a specific Pentacon Six mount plate, and if so, are they interchangeable across versions of this lens?
4. Any suggestions on how to remove this, I think the correct P6 mount is beneath this plate.

I’ve attached photos of the lens: flat screws and rear mount for reference.

Appreciate any insight — don’t want to force anything and damage the body or lens.

Thanks!

 

[Patrick]

Do you need an adapter?

 

[Medium_KV6C]

I'm unsure, it was advertised for the camera I have (Kiev 60/Kiev 6C) as a P6 (Pentacon six) mount, but I think its not the correct mount so I'm not even sure what I'm looking for!