MJM Maker's Mark - Diamond Earrings

Hi all, does anyone know anything about the below maker's mark 'MJM'?

 

MJM is probably the sponsor, because next to it is the London import mark, so it was made somewhere else.

 

I think it may be a Sheffield 1906-1998 import mark, not a London one?
I see the Greek letter omega with a horizontal line under it.
The date letter Q then corresponds to the 1990 Sheffield date letter Q.




London mark (apparently upside-down) vs. Sheffield mark:


Some Sheffield date letters:

 

Many thanks. And unless we have a specific country stamp, I'm guessing that we have no way of telling which country it was imported from?

 

So there is. I didn't notice that before.

 

AI says that MJM is the registered sponsor's mark for Mark Joseph Morrison, a silversmith in Sheffield but, oddly, I can't find any info on him at all.
HERE is a pair of diamond earrings, which are apparently also marked for Sheffield, 1990, and MJM (though the marks aren't pictured).

I think the only way to find that out would be by contacting the sponsor.