Welcome to the Forum....That is one of the RCA "Chamber Seal" guns from 1979,these had the #15 prefix, RST-6... It was offered to Members of the Ruger Collectors Association and I recall there were two series of number for the RCA 01400 to 01600 and RCA 1500 to RCA 1599, and you could have gotten them in consecutive serial numbers, the pair was under $400.....good gun, and if you have ALL the "accoutrements" then they are a bit collectible,so being around 300 total made, can be of greater value........hope this helps