According to the dates indicated (1891-96) & ( 1902) Hopkins & Allen could have made the Gun. H&A went out of business, bought by Marlin, in 1921. It could be possible that the KK was carried over, as was the case in many of these hardware guns. Could we also possibly consider that the butt plate was added after market, creating a "Jerry Rig" condition for collector purposes?
Correct me if this is incorrect, but wasn't Simons in business from 1875-1930? These dates would put it into perspective. Possibly a photo of a known Simmons butt plate would present the correct signature on the piece.