I would say when there is a noticable change in Design, if you were to take the Haro Master, there would be a lot of defferent models & some look the same until you look closer.
Take the 84 model, these all look very simular, but there are small changes.
Then there was a big change when the 85 Master came out.
But if you compare the 84 Master to say a late 80s early 90s with the Bashgard they don't even look close.