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?
holes can be explained as attempts to make the object more or less fragile and susceptible to breaking or lasting longer after bringing one horses down. A work in progress no one got exactly right and was discontinued.
Engraved ring might just be maker’s signature - a decoration if you will.
Manpads are more effective at taking out a tank then mines, but we still use both.