That's interesting.
I have a fired case similar to yours (it has a groove near the neck but not as deep) that came from an M2 Bradley in Germany.
I would guess there was a common round that was used in different guns in much the same way as small arms...if it works, why not.
I also have a pristine TP round, green steel case and sax blue head (TP) as opposed to drill (Oxford blue or unpainted).
Some drill rounds were not painted as the paint just got trashed or gunked things up.
I remember seeing 40mm 40/60 DP rounds that were natural with red grooves.
They often have grooves for ID, but I think in this case...the live case...has some sort of groove too.
Both of mine do.
Funny old world