The way I heard the story, everyone was running between 350 and 450 (with the exception of the 501, of course) for a long time, with nobody really going into 450 territory for the longest time except Maico with their 450 (also called 440 in later years, but they were always 438cc) until '80 when the YZ465 hit the scene. This was a big deal because until that time, the 450 Maico was the holeshot king and then they saw this new Yamaha as a direct challenge. So they got a little mad and drank a little beer in the German tradition and pulled out the 490, and the same year KTM brought out the 495. Those two stood alone as the near-as-makes-no-difference-500s in a sea of RM400s, CR450s, and YZ465s and the like. Then, the next year it was obvious everyone else had a displacement crisis because that's when the big boys came out to play, including the CR480 and YZ490. So it wasn't a linear progression as you described, it was really more of a jolt that brought everyone up in the course of a year or two.
This is all massively off-topic of course. But hey, I didn't start it!
