Two of the three letter marked bodies I showed above have Ulster base plugs, for which I do not question the provenance of the pairings. I have seen at least one other letter marked body with an Ulster plug, which again I believe to be an original pairing, and I agree the G marked body you saw also has its original plug.
In February I spoke with Dave when he got his R marked body (and incidentally it came with a 1-17 James Reynolds base plug, so an Irish connection there, albeit wrong body) and suggested then that the letters were codes for the score of firms making Mills grenades under the auspices of the Belfast Area Committee. However, while it is nice to believe this, it is not proof positive, though the more examples that turn up, the greater the support for the case.
All we can say at the moment is that it would appear from maybe four or five examples known to us, bodies marked with various letters might well be survivors from the Belfast contract.