Wayne the best way you would relate the diiferences would be a 74 is like a Tm 125 and 75 , 76 are like an RM 125S . Motor changed fork all but rolling frame is way superior , 75 & 76 different paint and colour mudgaurds.
Big K has summed it up well , motor aside they are very different bikes . The late Barry Senn told me the 74 KX 250 is the closest thing you will get to a 74 RH 250 and Barry had 74 RH, TM, YZ, CR and KX so he new what was what.
Where as i call my 76 a grown ups RMS because thats what its like to ride , an RMS with 50% more power
Both really cool bikes , if you got the chance grab either. I think the 74 is worth more as its pre 75 legal and rarer.