Bourne guns would vary due to the market demand. I'm not sure if he made them or sourced a lot from the trade and finished them. I have seen Bourne guns that were hammer guns made for the agricultural worker to decent hammerless guns with tasteful engraving. There are some serial number records in exsistance for certain number sequences. Also his address changed over the years. If you can give the number, top rib address and proof mark details I might be able to narrow the date down for it for you. A photo of the proof marks is the most helpful. Lagopus.....
