Help needed - What is this lens mount?

[micloi]

Need to buy a converter for a lens I bought but I do not know what mount it has.
I was told it was Olympus OM but I have an OM mount 2xTC and I cannot get the lens on it so I assume it is something else.
Also tried a Pentax-K converter, no go

 

[micloi]

The small recess shown on the image in the red circle might help identifying this?

 

[chris321]

What make is the lens? Some photos of the markings might help identify it?

 

[micloi]

It is a Mitakon 85-300mm

I had a better look online and I am leaning towards the Contax Yashika y/c mount

 

[Canon Bob]

....I am leaning towards the Contax Yashika y/c mount

Certainly looks like to me.

Bob