Officer's Model Special - Loose Rear Sight

[Winchester 88]

The rear sight on my Colt Officer's Model Special is loose. It can be wiggled slightly both horizontialy and vertically. This is a recent development and has negatively impacted the groups (scattergun). I understand this is a Coltmaster rear sight. All adjustment screws are snug. Before I take it to my very compenent gunsmith for a look-see, anything I should check? Thoughts?

[A1A]

It seems there is nothing that would be moved so often as to cause wear, unless it has been worked on previously. I would hate to suspect frame stretching. Unfortunately, replacement parts, if they are the fix, are scarce to non-existent to my knowledge, although there are a couple of slim possibilities. The Coltmaster was used for a very short period but also on some other models, so perhaps other members have some suggestions. Interested to see what your gunsmith finds.