Different oil pump, cover on the plug wire is different, the bar plates are lower quality too, they say the same crank, I oddly had three of the 501 saws and think there was a nut on the end of the shaft, could be wrong there though, had a few saws in that size.
They also don't have the P after the number like 501P had, echo seem to succumbing to corporate pressure
to produce cheaper saws to boost profits, but the 501 621 and 7310 are still pro grade, lets hope they stay
that way, I would say they are a good saw, but if your going to run it every day I would go for the 501P.
Even in their advertising the small saws like the 352 were described as pro built, don't see this in the newer
model descriptions. But I doubt too that Echo would make a bad saw, that is how you loose customers and profits.