My guess is that much of the reeding was sanded off but could be wrong! Measure the diameter and if the coin is much less than 17.9mm, then that is the case.
It is PMD. The missing reeds on coins can happen from a number of reasons. For example, when you handle casino coins, they are practically smooth from their frequent use. Another example is the instance where coins are stuck in a dryer. They are called dryer coins.
My guess is it's a coin machine. If a coin rattles around in a machine without going in, the reeded edges get cleanly polished off. They look a lot like that.