[Tom E Gun]

I think the BX-15 designation refers to the Ruger 5 round magazines:

http://www.outdoorgearbarn.com/p-17814-rug-90041-mag-bx15-1022-5rd.aspx#.VJyBmncBoEA

http://www.tombstonetactical.com/catalog/ruger/90041-magazine-bx15-1022-22lr-5-rounds-black/

Ruger 90041 Magazine BX-15 10/22 .22 LR 5 Rounds Black