We've previously dicussed the symbol Eckhel used meaning the 'reverse' of a coin. I've found a new candidate for it in Joannes Dominicus Coletius' Notae et siglae quae in nummis et lapididus apud Romanos obtinebant explicatae [= Mark and sigils upon money and stone according to Roman explanation] (1785).
Coletius' list, which spans pages 421-445, includes symbols like the Roman Denarius Sign and Roman Sestertius Sign, recently added to Unicode in the Ancient Symbols page at code points 10196 and 10198 along with bizarre symbols I've never encountered.
The first symbol on the top of page 439 looks a great deal like the version Eckhel's printer used. Coletius explains it as 'Quinquies'. This word is aparently the 'numerical adverb' for the number 5 ('quinque'). Oddly, although Google believes there are 423,000 web pages with 'quinquies' but it is missing on most online dictionaries of Latin.
If this is the symbol, it remains unexplained why the symbol for '5' would be a good choice for the reverse of a coin, or where the practice of using it started.