Thank you Adam.I'm going to guess it's the AT code, so March 1970. The other options for T are 1926 or 1999. The AO looks like larger font than I'm used to seeing for date codes, but maybe someone else can confirm that or not.
90% sure E prefix is much later receiver.The receiver is numbered E6548###.
Another clue is the antibind cut in the bolt lug. I have a 1968 mod and the antibind cut does not exist. That was from the left over 722 actions when the 700 was introduced. The slot was showing up by 19701968 and later the shroud was extended so the firing pin was recessed when decocked. 1974 later had a one piece sear connector(? left side).