BirdHunter,
You have a very unusual Beretta. The 408 Mdl makes more sense than 409. I have and have had 409s from 1936 to the 1960s and none have had the special feature you have on yours. The top rib, the forarm button, and the third bite are all different than those I have seen. The 408 could be a fore runner to the 409 or a prototype. In any case it is different and special. Try this person and ask what model this SxS is. marilia.santos@beretta.com...Marilia may have an answer for you. She has helped me in the past.
Best,
Ron