The RTO5 and RT501 codes seem to overlap a lot, and both be in use at the same time.
RTO5 Imperial Army - WD96 (Dec 1987)
RTO5 Imperial Army - WD96 (Dec 1987)
RT501 Imperial Army - WD98 (Feb 1988)
RTO5 Imperial Army - RT1 Flyer (Feb 88)
RT501 Imperial Army - Astronomican (Feb 88)
RT501 Imperial Army - Astronomican (Feb 88)
RT501 Imperial Army - RT2 Flyer (Mar 88)
4010 Imperial Guard - WD109 (Jan 1989)