The marketing term for early side plates was Ornamental Strengthening Plates.
I think Cogswell & Harrison may have invented that term. And they made a lot of guns with the little added metal plates right at the head of the stock, behind the receiver. Plenty of Hellis doubles with that same feature. And Mark just sold me a nice Alex Martin 12 bore, #2 of a pair, with those added plates. Can't recall ever having seen them on a Martin before.
Bernardelli (and Francotte) both did one piece sideplates that made the guns look a lot like a sidelock. They do provide more space for the engraver to do his handiwork.
