[Austin J Hawthorne Jr.]

Not wanting to hijack Bruces thread I thought I'd start another one. Among the various Birmingham proofmarks on my 1881 W.C. Scott Hammergun is one that I can't identify. It is a Crown over an S.
Also, on the barrels, are the marks...16B 18M on the right barrel, and 16B...17M on the right barrel. Would the (M) marks indicate choke ? All these proofmarks are on barrels (not the barrel flats). Can anyone familiar with British shotguns help ?
Thanks in advance.