How about a helmet as a symbol. I had trouble deciphering the symbol. This symbol is not listed in Crawford, Grueber, Sear or my other RR books. http://rrdenarius.blogspot.com/2015/11/q-titius-as-90-bc.html
Not a helmet exactly, but the apex worn by the Roman priests known as flamines is certainly in the running as the most unusual headgear depicted on an ancient coin. This was a pointed piece of olive wood, the base of which was surrounded by wool. This was then attached to the priest's head with string. This contraption most resembles a lampshade worn by a New Year's Eve drunk:
Wow @Pishpash! That is a mighty fine bronze Macedonian shield. Here is my only helmet from Mesembria. Thrace, Mesembria AE17 Thrace, Mesembria. Circa 4th-3rd Century BC. Æ 17mm (3.85 gm). Crested Corinthian helmet facing / META in spiked wheel.
How about this Severus Alexander sestertius with a snappy looking Mars Ultor? The reverse is what drew me to acquiring this coin, plus at the time I did not have a sestertius.