I went looking for my notes on this subject and searched and searched the blog and couldn’t find the post I wanted. Only to realize that my notes were pre-blog! So for future easier searching here’s a collection of factoids related to RRC 293/1 (c. 113-2 BCE).
The most important coin type for understanding this type is the earlier RRC 259/1 which uses the same style helmet as a reverse symbol by a moneyer, Q.PILIPVS (c. 129BCE).
Textual evidence is clear that this helmet is associated with Macedonian Kings:
Livy 27.33:  The report was current that Philip had been killed; the rumour was due to the fact that in the encounter with the plundering parties from the Roman fleet at Sicyon, his horse flung him against a tree and one of the horns of his helmet was broken off by a projecting branch.  This was afterwards picked up by an Aetolian and taken to Scerdilaedus, who recognised it. Hence the rumour.
And not just for Philip V:
Plut. Pyrrh. 11.5: the greater part of the army was all excitement, and went about looking for Pyrrhus; for it chanced that he had taken off his helmet, and he was not recognised until he bethought himself and put it on again, when its towering crest and its goat’s horns made him known to all.
I’d even argue based on these two coins that this gem (in Vienna) was originally created for a Roman with the cognomen Philippus.
Of course the weird thing is how little this intersects with Philip V’s own self representation or really that of any of the Diadochi.
One can point to ibex horns on Seleucid helmets, but visually these are not strong parallels.
It’s also very different than the standard numismatic representation of a Macedonian helmet:
The thing the Roman representation looks most like are the horns on the head of Pan on obverse of Antigonus I Gonatas’ tetradrachms:
Is the helmet emblem of the Roman Philippi derived from one big iconographic misunderstanding?!
It has been suggested that the head of Pan may be intended to be a portrait:
Why the cognomen Philippus? Briscoe gives a good overview in his commentary on Livy.
Fuller gem info thanks to Dustin McKenzie Jan 2020