Well, maybe these things were a bad idea in the first place, but if you’ve ever ridden a horse, then you know their footing on lose rock is not so hot. So the more unstable the ground, the harder it is for horses to make speed - particularly a full on calvary charge. I see this as the best option for a military use by the Romans.
Broken pottery does not roll and is fairly stable, compared to a multi-surfaced hollow object with knobs.
OK, lets assume that the the Roman military industrial complex convinced Caesar that the equivalent of a rifle round with the price tag of an ICBM was a good idea...
How does the changing the size of the holes on the sides better injure a horse? How does the changing of the engraved ring around the hole cause the horse more difficulty in battle?
Wouldnt the caltrops in a clay oil flask grenade like the Greeks used be more effective and cheaper?
The Romans typically won battles, and these are mostly found in old camps, but not in any great numbers in any one place, apparently. So, could be, but more likely some kind of amulet or charm, because by the time of the Flavians the Roman army and indeed the Roman Empire itself was mostly Romanized non-Romans. :^) The cult of Mithras was known as the soldiers’ god, and originated in ancient Iran.