Pics do the job !. Firstly are you sure these pics are of a 12b, the second pic shows proof marks of a .410 ?. Anyway I would say that the gun was originally proofed, probably for Black Powder only, some time prior to August 1904 and was subsequently re-proofed for Nitro sometime between 1904 and 1925.(The action flats show the pre and post 1904 View marks).
Maybe not relevant to the original question but the wood looks like its been replaced with a qualty piece of walnut. Finally is the r/hand cocking dog misaligned in some way ?