BSA (Birmingham Small Arms) was the largest private arms maker in the UK during World War One, and when the war ended it of course saw its huge military contracts evaporate. One of BSA’s efforts to develop new markets and product lines was to devise a series of self-loading pistols. These also involved the use of a line of new belted rimless cartridges made by Kynoch. Very few of there pistols survive today, and this .45 ACP example is one of them. It is a short recoil, rotating barrel design, with a trigger mechanism very similar to the FN 1910, and the rotating barrel system very similar to the Steyr-Hahn 1912.
The best (and pretty much only) source of information on these pistols is an article written for the HBSA by this pistol’s former owner, Dr. Geoffrey Sturgess. The Julia auction house has thoughtfully provided a copy of the article in PDF format for folks who would like to learn more.