Daniel

I've never seen an "O" like that before. The fitters marks were
always placed around the stock pin, in a rather uneven pattern.

If you have higher-resolution copies of the pictures showing that
"O" on both the barrel and the grip frame, and also a good picture of
the serial number on the butt, you should set them up in a library
here, and make a posting on the S&WCA part of the site, if you are a
member. (If not, I can do it.) Perhaps Roy will comment on that
mark.

Mike Priwer