Originally posted by JohnX Even without the key, I've never failed to remove an M42 adapter from a mount.
There is a first time for everything...
IMHO, unless the box says Pentax or SRB Giturn, the most prudent action is to remove the retaining spring. Do a little research on this site and you will find plenty of material on stuck adapters including a few where resolution required use of a Dremel rotary cutter.
Steve
...once spent 3+ hours trying to remove a brand new adapter that stuck on its first mount to camera...finally succeeded...sent back for refund...lesson learned...