STX-RC made it's debut in 1995. It was produced until 1999 but rec'd an 8 speed upgrade in 1997. It was 4th in Shimano's hierarchy of off-road groups behind XTR, Deore XT and Deore LX. It replaced STX SE which was a 1994 only group that replaced Exage ES.

Personally, I would go with a current Shimano Altus derailleur. They're pretty much the modern equivalent of STX, having aluminum upper and lower pivot housings mated to steel parallelograms arms and cage. Thye're also pretty inexpensive at around $25 and should be readily available through any LBS. Alternately, you can always try to source something C&V, pay the shipping and wait for it to arrive.