Tony,
FWIW my understanding is that the 77-78 RSGS model had straight swingarm & shocks laid down at about 45degrees as per pic in NSR's reply #3 above. The 250 also had the short stroke (57.5mm) engine.
The model w more upright shocks, banana swingarm & rearset # plates (as per top pic in NSR reply #1 above) is 80 TF1 (tho they may have been available in 79).
My 250 frame # was *AUS*SWM*RS250GS*E*0007 & I was lead to believe it was a 78 model, which was almost exactly the same as 77 model. (Its now being raced in pre78 VMX.) My Eng # was 87577 & it was short stroke
The long stroke 250 engine (61mm) was first used in 78 on Canam MX4s as far as I know. So it seems to me your engine is a later model transplant (which is perhaps why you can't find an engine #). I believe they are not pre78 legal.
Whilst your frame is obviously RS175GS, it appears to have a later TF1 swingarm