At a very high level, skipping things like air filers and side sills) here is a breakdown of GP38-2 spotting features. This came up in my research because MoPac bought GP38-2's almost every year from 1972 to 1980.
81" short hood, wide spaced fans, long radiator grill (like a GP38), I think EMD made these through the end of 1972 or so. Atlas makes this version.
81" short hood, close spaced fans, with the grid type radiator grill. I think EMD made these from 73 to 75 or so. Atlas makes these as well.
81" short hood, close spaced fans, corrugated radiator grill. I think these were made in 1975-76, I can't recall if Atlas makes this version.
88" short hood, close spaced fans, corrugated radiator grill, EMD made these from 1977 to 1980 or so. The LL/Walthers model is this one (the one with painted on sight glass)..
88" short hood, close spaced Q fans, corrugated radiator grill. EMD made thses post 1980, I don't think anyone maes this version either.