looks like the real thing to me. I have one, also BB-prefixed serial number. Skennerton's book states, "in all, 50 No.5 rifles were ordered with the triggers mounted or hung from the body, converted from No.5 Mk1 models..." Somewhere here on the forum, someone suggested that number may have been as high as 150, but I cant confirm that. Cant really give you a value either, but I bought mine 7 years ago for 2K. Yours is the only the second one I've seen. Vey nice rifle you picked up!
Forgot to add: when I purchased mine, I used the "buy it now" feature on GB. It had posted for only a few hours when I jumped on it. So had it gone to bidding, who knows how much $$ it would have brought.