Very old indeed!
Roman Empire, Diocletian, 285-305 AD (probably minted after 300)
Silver plated follis, Rome mint R-crescent-P
Moneta standing with scales,
SACR MON VRB AVGG ET CAESS NN
"Sacred money (of the) City (of Rome) (issued by the two) Augusti, and their (two) Caesars, our Lords"
Similar example with a different mintmark
https://www.acsearch.info/search.html?id=61764Sadly, value is minimal, just a few dollars. These are common coins from a common emperor, and are often available in very high grades.
I had the same type of Constantius I, father of Constantine the Great, who was one of the two Caesars (deputy emperors) at the time:

That one sold for $18