Hi Burnieboozle,
Welcome to the Forum.

I thought you were never going post those images of what sounded like a very interesting ring.
I think it is likely that this is one of the marks used by Ferguson Brothers (James and William) of Inverness:

Ferguson Brothers - Inverness - 1873
The 'P.M' mark is a mystery, for there is no-one of those initials recorded, to my knowledge, working in the trade at Inverness at that time. However, there is a possibility to the identity of 'P.M'.
The 1871 Inverness Census records a Paul McBean, a 21 year-old lapidary apprentice. The name McBean is important in this case, as William Ferguson was later to go into partnership with his brother-in-law, James McBean, as Ferguson & McBean, so it is possible that Paul was part of the same family and was employed at Ferguson Brothers. Although it would be unusual for journeyman to mark an item in such a way, I don't think it would be impossible. But this is only speculation on my part and only offered as a possibility.
Should Paul McBean be the maker of the ring, then a clue to what he may have looked like might be found in the image of William McBean, a Melbourne jeweller who is likely to be part of the same family.
See:
viewtopic.php?f=38&t=18484&p=59709&hilit=mcbean#p59709Trev.