The proof mark appears to be the preliminary proof mark given to the tubes prior to assembly into a bbl set. This would go along with your thoughts as the finished gun would not have been in England for final proofing.
Incidentally, I have a J P Clabrough, & while certainly not a Purdey, Boss etc I would not call it a ""cheaper "put your name on it" (ie Clarbrough sp?)guns from England"". It is a quite well made gun indeed & from an 1895/96 Clabrough, Glocher & Co catalog varied in price from the basic non-ejector field grade @ $50.00 up to their best ejector @ $250.00. This cheapest grade is priced, from the same catalog $2.00 under a PH Parker & $5.00 above a quality 1 L C Smith. The cheaper "put your name on it" guns are about half or less of this price.
