I agree that it looks like a 1927/8 OHV BSA but the strange thing is that the oil tank does not appear to be in the fuel tank. There are not two filler caps in the tank. It appears from the photos that the oil tank is under the saddle, tank cap missing, where the tool box should be? Not seen any thing like this on any pre 1930 BSA. I won't give an approx value as I don't want to held responsible and also doubts about the originality as per the oil tank.