Matt M. is correct.
RC 4 Pure - RC 4 decoder only
RC 4 Hybrid - Any AMB decoder system made to date.
MRT - Decoder Rev 3 or older including AMB 20s.
I'm going to get ready to send in all but one MRT to trade for the RC 4 Pure and clones (so I've read). If I can't get clones, I guess I'll have a bunch of singles. Each trade in will still cost less than buying 1 new PTX without a trade in as long as the offer is in place.