I have a REX chip also and it's listed in CashCoinConnection (John Kallman/late Dick Staeffler guide) as the Rex in Sacramento. Good enough for me unless someone knows otherwise. Odd thing was, the chip was sold to me as an AEX Crackerbox chip (that R looks a bit like a bent A) and that was a bad identfication.

Sorry, no such luck on your MRB. Another Jay-Mystery chip, for now.