"F" and "E" on my oil filler cap? Why?

Okay guys, looking for the best slant six minds here. I own two 74 Dusters, both with the slant six motor. On the oil filler caps on the valve covers, there are two letters stamped on the left and right ears of the caps. The left ear has an "F" stamped on it, and the right ear has an "E" stamped on it. Almost like it was referring to "full" or "empty". What do these letters mean? Any ideas? It's not referred to anywhere in the owner's manual or the shop manual.