Ted, anything that came with (and still has) drum brakes, air-cooling and non-linkage rear suspension is an Evo bike.
Depending on the race organiser, they usually run capacity classes for the Evo bikes. Typically, the 250s and opens get plenty of people fronting up, but fewer 125s.
I should add that I don't disagree with the statement that the YZ250G/H is one of the better Evo 250s out there. I wouldn't trade mine for anything... except a better YZ250G... or some riding ability...
I'd say that the 'stand-out' Evo bikes would be the 79 CR250s, 80/81 YZ250/465s, and the 81 Maico 490s, and maybe the RM250/400N/T, but I think the original question was about specific bikes.