Amongst other bits I have the ring making kit, it recommends making it 10% fatter / thicker, and 10% larger internal ring size to that required for the finished fitting. 995 is definitely softer than 925, but are you going to use the pen as a mallet?
I haven't looked for a while, but Reputable suppliers, like the one I linked to, used to offer a hallmarking service.
Your finished article could be returned to them, and for a set fee, they dealt with the legalities, it was assized, marked, and returned.
PMC is normally engraved with a laser hallmark, as stamping is not advisable.
