The BSS was made by Miroku in Japan for Browning. Even though the guns are long out production, there seem to be a lot of parts floating around.

Here are some sellers with Miroku parts on gunbroker that might can help. Try contacting them to see if they have what you need.

http://www.gunbroker.com/Auction/ViewItem.aspx?Item=483704908
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=483564872

Good luck

Adam