Brian - Let me know what your gun's London address is, and I may be able to ballpark a manufacturing date. Cogswell & Harrison made most, if not all, of their boxlocks (side-plated or not) in their London factories - but these have been moved from time to time. I have a side-plated Coggie boxlock built in 1906 at their Gillingham Street Works near Victoria Station.
The Konor, being made in the '50s, just after WWII, might possibly be an exception; I have e-mailed the current incarnation of C&H to get an authoritative answer, and will post if and when they reply.
The rib reads
Cogswell & Harrison Ltd. London 168 Piccadilly & 226 Strand
SN.55xxx
It's a non sideplated, assisted opening, ejector boxlock.
