The oval in the stock had an oval piece of metal( Brass, Nickel Silver, Silver ?) let into it, that may have had initials engraved into it. The initials stamped into the stock are a little unusual, but may have been placed there by the workmen that did the stock work and checkering. The stock may be a special request. The 12 in a circle means it has 65mm( 2 5/8") chambers, the 13 is the diameter of the barrels( expressed in gauge measurement) ahead of the chamber. The left barrel is choked( as shown by the crown W), but amount of constriction is not shown. The crown S means the barrels were proofed for shot. The crown U, with an eagle over it means it underwent a View proof, after undergoing a definitive proof with the provisional proof charge. The view proof is a detailed inspection, including verification of dimensions. The Shul proof house started dating them in the early 1920s and this one doesn't show a date, so it was likely made before 1923; The type of proof marks indicate it was proofed after about 1911. The Large crown and "Wildman" stamps are not proof marks, rather they are JP Sauer and son house marks. Whether you have the chambers lengthened or not is entirely your choice. Some people shoot them "as is" with 2 3/4" shells, some people have long forcing cones reamed and use 2 3/4" shells, some people have the chambers lengthened, some people buy short shells. Most people have a strong opinion, one way or the other; but I'm not aware of any damage caused by any of them.