MIKE GEORGE says:
On a sidelock shotgun, the lock mechanisms are built on separate plates which are fixed to the wood of the stock immediately behind the action.
These plates carry the hammers, mainsprings and sears.
A side-plated gun is a conventional boxlock, with all parts of the firing mechanism mounted within the action body.
However, plates giving the appearance of artificial lock plates are added. These plates are merely cosmetic in function, but they do provide a large area to carry elegant engraving.