Hi,
this is not really RPN like in the older HP calcs,
but a simplified version, more like an adding machine.
These Minuteman 6 calcs were made in different versions,
some with and some without an ENTER key.
AFAIK only the Minuteman models with a star (*) or cross (X)
are (simplified) RPN.
Some used 9V battery blocks,
and some used two 1.5 V batteries, IIRC.
Commodore (and some other brands) also made
some bigger machine(s) with real RPN logic.
Raymond