Springfield/Stevens 16 ga double barrel markings??

[Raybud]

I just got my Dad's old model 5100 double barrel. I think I've dated it to 1952, (letter D in a circle under the breach), however there are several other letters there as well. Anyone know what they might mean? They are: G, F and E.

 

[Raybud]

I took this out and capped of a few rounds today. Now I remember my Dad saying that it kicked like a mule. I'll be bruised tomorrow.