I just ran across this thread, I have two of the box magazine BSA's - both under 1000 serial number; Unforunatly I do not have a magazine for either of mine and in several years of looking and asking I have not been able to locate one. If anyone runs across a source for an original magazine for one of these I would really appreciate a chance to get one. I currently have an aftermarket Winchester Model 77 magazine that I am contemplating trying to modify.
I also have two to of the tube fed with one having a serial number over 6000. There was a tube fed sold through Christies Auction House that had a serial number of 7656.
There is some good infomation about the BSA's here - BSA pump info
Note that one picture shows the serial number and it appears to be 1077 so it's my opinion that over 1000 were made. I also found copies of the Britishpatent papers on these through a British Library web site but the saved link no longer works and I now cannot find the resource - they had British patents back to 1920.
I am going to compare proof makes but I assume that all of the box magazine versions were made in the same year. I assume that the tube fed could also be dated by the proof marks?
Gene