The EE3 had a long production run. From May 1973 to October 1986.
Behind the pressure plate there should be a production code. (back side of pressure plate, that is)
Such as 'S0X5' (letter S, number 0, letter X, number 5)
'S' plant code
'0' last digit of year 1980
'X' month October
the 5 is the pressure plate type
the months run;
This information is from "Olympus Pen Viewfinder Cameras" published in 2000 by Biofos Publications and written by John Foster
Another note, John has a website, biofos.com, it is worth a visit.