That most certainly is NOT a PK mount. Interesting - I would have said it's old olympus OM mount looking at the square black plastic release button on the side, but what's that large lever on the side? Exacta's (and Topcon) have those...
In any case I think you are a bit SOOL, you are not going to find an adapter for this to PK (
easy adaption to canon, mirrorless..). OM's can be modded a little by shaving the bayonet flanges to squeeze on to a pentax and that works reasonably because the
registration distances are just about the same. Exacta however has a much shorter registration distance, so you will never be able to focus to infinity. The only alternative is a diy mount swap, normally not worth the effort - best to just look for another example of the pens with the right mount - however if you want to try, a mirror lens is the easiest type of lens to try with because the aperture is fixed, no iris or aperture connections. Acquire a pk mount off a junk lens and figure out removing the current one and attaching the pk.