Is This Vespasian Legit? Unique Reverse

There is no legit reason to question this coin. The reverse with COS VII and eagle might be "unique" to Vespasian; but it is rated as common in RIC. This reverse was also issued for Titus with COS V.

The core "problem" with cataloguing types and varieties of ancient coins, compared to modern ones, is evident when one considers the differences in minting technologies between the two. A modern coin is made using a standardized, machine-replicated die, so that millions or even billions of functionally identical coins can be churned out, using tens of thousands of identical dies. An ancient coin is made my hand, and the dies used to produce them are also made by hand. In a very real sense, each die is a hand-crafted work of art and coins made from that one specific die are often classified by the cataloguers as their own unique "type" or "variety", depending on how the lettering was spaced etc.

Obverse and reverse hand-held dies wore out at different rates, with the "hammer" (reverse) die tending to break faster than the "anvil" (obverse) die. "New" reverse dies would therefore very frequently be matched up with "old" obverses, and occasionally vice-versa, thus the concept of a "mule" doesn't really exist as what we today call "muling" would happen all the time.

The next factor to consider is mintages. Obverse dies only lasted for about 20,000 coins before needing to be replaced; reverse dies might only last half that long (and even less, for harder bronze coinage). Mintages for each die pairing would therefore be quite small, probably in the tens of thousands at most. So each "type" might have had an original mintage of just a few thousand. No detailed records exist from the ancient Greek or Roman mints documenting these facts; this is all educated guesswork based on archaeological evidence of the coins themselves and from the few smatterings of mentions of coin minting in the surviving records.

Now consider survival rates. We can only guess at survival rates based on archaeological evidence, but it's not unusual for any given "type" to be known from only a tiny handful of surviving examples, or even just one. So it's entirely possible that even today a never-before-seen type is freshly dug up out of the ground.

There's no such thing as a "collection of coins" that's been sitting around as an intact collection since ancient times; the oldest coin collection in the world is in the Vatican and it was initially assembled in the 1400s. And if the ancient Romans ever compiled anything resembling what we might call a "coin catalogue", neither it nor any mention of it has survived. So everything we know about ancient coin types comes entirely from examining the surviving examples we've dug up out of the ground in the last few hundred years. There are, inevitably, gaps in our knowledge.

I had always taken "mule" to mean pairing of dies from two distinctly different types, not variants of the same type. But that takes us to the thorny matter of defining types.