Origin of RT-11 emulation on RSTS

I've wondered this for a long time and maybe someone here can shed some light. Back in 1975-76 I was using the PDP-11 and Oregon Museum of Science and Industry (OMSI) and they had a thing called "OMSI RT" on their system. It was an RT-11 run time system (RTS) under RSTS. They were selling V1A for $2350 a copy (I still have a copy of the on-line help file giving the information). The product ran the Fortran compiler, MACRO, Pascal, etc., as the later RT-11 environment from DEC did. OMSI RT claimed compatibility with RT-11 V2 and also required a DEC binary license for both RSTS (V5B or later) and RT (V2) My question is: did DEC buy this from OMSI to (improve?) and include in RSTS or was the DEC RT-11 environment created some other way (either developed in-house or purchased from someone else)? Does anyone know the origins of the RT-11 emulation under RSTS? Thanks, John