the 175 version is basically 79 pe engine different carb and flywheel in pretty much rm frame
They were all pretty much an RM frame - just which RM is where it got interesting.
'79 - NO. The 1980 RS was the first year. The PE 1979 C/N engine is quite different to the T/X/Z engine. The RS is the T engine with bland porting, lower compression and 30mm carb instead of 34mm. The pipe was altered to suit, so avoid using one on a PE! Also the RM100T steel swingarm, not aluminium like the T/X PE's. Even the hubs/brakes were the smaller RM100 ones, not the PE T/X ones, I think.
The RS250 was in a similar vein. The funky RS handlebars, an RM tank, stifled carby, stifled porting, same forks as the RS175, special softer rear shocks, the brakes off earlier models, and the rear wheel was DR400.
Note the DS185 headlight and tail-light. Parts-bins-R-us.