Two models; a two stage and a rapid fire version. I have the two stage (B-G2S-E), and it is a great deal for what you get.
https://www.brownells.com/aspx/search/productdetail.aspx?sid=138461&pid=67223
https://www.brownells.com/aspx/search/productdetail.aspx?sid=138461&pid=67223