The maker, build quality and action used can be complimented greatly by quality engraving.

I'd imagine that a best quality piece engraved by Harry Kell or his Continental equivalent in the 20s-30s would bring top dollar plus, maybe plus 15%.

Rember that in past times the best engravers were principally working for Jewellers and watchmakers.

For the gun trade engraving was the least renumerated trade and usually done by outworkers, thus what you see is what you get (the tiger/big pussycat syndrome).

More recent pieces from the 70's when engraving drew a more informed audience then maybe 25% or more.

For boxlocks then maybe 10% absolute tops. (unless maybe a WR)

Currently a ne plus ultra Piotti with a top name that complimented the piece then maybe 33%.

But this is drawing a broad brush. Horses for courses.