Using a magnifying app on my phone it is RB or BB if you are talking about the letters in the crossed swords. I know very little about these guns.
Yes, we are trying to figure out what the letters on each side of the crossed swords are.
RB would be 1966, BB would be 1951. 1951 is not consistent with the other proof marks.
The stamp on the barrels does indeed look like an S but even magnified can’t make out the other letter. When I loaded the photos into ChatGPT it dated the gun mid the late 60’s.
It's almost got be a B, based on the other marks.
A C is
possible, but that would put the mark in 1991, long after Webley & Scott stopped making shotguns around the end of the 1970s...and the rest of the proof marks do not align with a 1991 mark.