Dave - the 39 BSA (Birmingham Small Arms) was part of a series, and was very limited production. Original rounds had no headstamp, and are very rare. I have a replica, headstamped, that Kynamco made some time ago (I was told it was made by Kynamco at any rate). Even those are scarce and it was expensive. I bought it only because I had none of the series in original rounds. The other two were the .28 BSA and the and the .34 BSA. Brief Descriptions and Photos can be found inthe book “Handbuck der Pistolen- und Revolver-Patronen, Band II,” by Erlmeier and Brandt, as numbers 316, 339 and 380 (pages 48, 77 and 132).In general shape and proportion, they have the look of a belted 9 mm Browning lOng cartridge. I am talking only of appearance. The case length of the .39 BSA is actually around 22.95 mm (.903 Inch) with a true caliber of 10.13 to 10.15 mm (.399 to .400 Inch).
I believe replicas were made of the .28 and .34 BSA rounds as well as the .39. I remember two different being available when I bought mine, but I could only afford to purchase one of them, and picked the one most commonly found as a genuine specimen as being the most representative of the genre.
Hope this is of some help.