Christian I'm basing this on a buddy of mine who's main business is Echo repair. He has educated me greatly. I owe him.
He tells me the three colored "bugle" logo early 70's are the best EP-3's. Up until 74, the kept making changes to improve. After 74, changes stopped being for the better.
Post 74: Started trying to save money. Used particle board housing (formerly plywood), switched motors to cheaper & less rugged, chassis made out of aluminum, used nylok screws, boards more fragile, lower resale value. Couple of years they buggered up transformer placement. One of the bottom screws had to be ½" shorter and if you mixed them up, the longer screw punctures the transformer. Bias board harder to get to. Probably more but that's all I remember.