In response to EOD, here is a sample of dummy and inert US 7.62mm NATO rounds from my collection. It’s not the most comprehensive lineup but there are a few variations:
Dummy, T70E4 (technically not NATO at this date)
I have several dates for the M63: this is the earliest one I have while the latest I have are dated 1960 (both FA and LC with NATO symbol).
The following four inert rounds seem to be variations leading up to the M172, which is pictured last. These all have an inert filler to simulate a powder charge. I’m not sure when M172 became an official designation so I won’t label these with a model number.
(I also have the round above with a (+) FA 60 headstamp)
The latest date I have for the M172 is (+) LC 69.
I have several other dummies with US military headstamps and later dates, but as Ray mentioned these seem to be unofficial dummies made with existing cases.
In particular, I have this one ((+) LC 75) but I’m not certain if it is military issue (it has a blind primer pocket):
I also have this round but it was reportedly made by Dillon to function test their miniguns, so it wouldn’t qualify as an official military dummy: