"A strange gun of obviously alien origin. It can support small energy cells, however."
I suppose it can be taken as the Vault Dweller was just assuming it's alien in origin, but then this line is thrown in: "it can support small energy cells, however." That in spite of the fact that it seems alien, earthly components fit in it/work with it. So, it could not be alien at all, but there is the possibility that it is, in fact, alien. (Yes, I'm aware that that line is in there to let the player know the ammunition, but still it can suggest more than that.) Or the best explanation of how it can use SECs is that its just an Easter egg.