We have had this in the family for a long time (60+ years) and no one knows what it is or if it is real. Any clarification would be greatly appreciated. And whether real or fake, what are the markers that indicate such. Thanks in advance.
It is Byzantine but I don't have the expertise in that area to tell you much more.
It is indeed Byzantine Empire, so calling it "ancient" may not technically be correct. The "XXX NNN" inscription is a pseudo-date, a legacy from when the Byzantines used to speak Latin and use Roman numerals; on earlier coins it would have said "ANNO XXX" (year 30) or something similar; by the time this coin was struck, everybody in the Empire spoke Greek and nobody who was involved with coin-making remembered what "ANNO XXX" was supposed to mean any more; it had been reduced to meaningless decoration. The large letter "M" is the Greek numeral for "40", indicating this coin is a follis, or 40 nummi.
The coin dates from the reign of emperor Leo III (AD 717-741), probably struck during the later part of that time period. The two people depicted on the obverse are attributed as Emperor Leo III on the left, and his son and co-emperor Constantine V on the right; the obverse legend is partially legible and is supposed to read LEO S CONS, or something similar.
Your coin is what ancients collectors would call "overcleaned"; Byzantine bronzes are normally covered in a thick green patina and a coin that's been scrubbed down to bare metal is considered less desirable. Here's a similar coin in better condition on Numista: https://en.numista.com/catalogue/pieces134225.html
Sapyx, thank you for the thorough explanation. Is there any way to tell if it is a "counterfeit" It weighs about 1.4 grams less than the example you pointed me to. I imagine that, as those old coins hand-made, there would be some variation in weight, so does that lower weight signify?
Thanks again for the help.
Ancient and Byzantine bronze coins had much less strict weight control than gold or silver, so I wouldn't worry about it. This particular coin type is even worse than most; the catalogue mentions that they are "often found restruck on earlier coins", so as well as making brand new coins, they've taken random older coins (of various weights and sizes) and recycled them by overstriking.
The corroded-and-cleaned state of your coin may also explain some of the weight difference.
In general, for a coin like this one, I wouldn't worry about the possibility of a counterfeit. Counterfeit Byzantine are far less likely to be encountered than Ancient Greek or Roman, and when they do occur, they're usually much more identifiable than this one. You're probably more likely to encounter an early Islamic imitation of a Leo III coin than a modern counterfeit of one (and no, yours isn't an Islamic imitation, because those normally have the Christian crosses deleted).