It's been ages since I was a regular, but wasn't there a member c. 2010 who was really into the Ptolemaic series?
You may wish to compare your coin with similar
types on the
ptolemybronze.com
web site.
Diameters and thicknesses of
Ptolemaic bronze coins are highly variable (on a single coin) and useful only as ballpark vague size ranges. The coins are seldom truly flat or round.
Weight is the only reliable size measurement for
Ptolemaic bronze coins.
Afaik there is no such thing as a
Ptolemaic bronze coin that is 15mm thick - not even close.
The
Ptolemaic bronze tetrobols
average weight is around 45 grams and are typically 35 - 40mm
diameter (and may show that range on a single coin).
There are several
types of
Ptolemaic bronze tetrobols. Those of Series 3 (like yours) are a little more
scarce than the *extremely* common tetrobols of Series 4 and Series 5. All are about the same range of sizes and
weights. Interestingly, they are fairly worn so they almost certainly circulated a lot and were used despite their seemingly inconvenient
bulk. They were produced by at least five mints in several designs according to the same
weight standard, on and off over 50 years, starting about 266BC. The Series 4 and 5 tetrobols are so common that they're always available - impressive, interesting coins for collectors.
Enjoy your new treasure.
PtolemAE