It's just a matter of how old the rifle is which receiver it is. Yes there are E receivers and the so called old receivers. You can find a list of the differences between them on this web page
. There was no great advantage of the E receiver. Savage was using them on the 93's already and they just decided to put the same receiver on all their rimfire rifles. The main difference you would notice is the E receivers are drilled and tapped for scope mounts while "most" of the old ones weren't.