Part # = 45100-40600-48F
Part Description = SEAT ASSEMBLY
Model Count = 6
PE250T 1980
PE250X 1981
PE250Z 1982
PE400T 1980
PE400X 1981
PE400Z 1982
To use RM400/250 seat you will need the RM fuel tank too. I reckon all you need to do is pop rivet the brackets in different spots on the base. In all the original RM250/400 photos ive seen, the RM, seat foam looks a different shape to the PE. The RM foam looks lower and narrower at the front and the PE foam is more of a square block.
Basically if your missing a seat and trying to find a 400 seat and wondering if RM is the same, well its not, just stick to looking for a PE250/400 seat which would be easier to find than a 79/80 RM seat anyway.