Model numbers should start with either M, N, P or F. Are you sure you are looking at a model number, and not the serial number? They are not the same thing.
Model numbers are 8 characters long plus a forward slash. Serial numbers are 12 characters long.
The model number should have the forward slash before the last character.
MWC62AA/A for example.
At this point in time, it's highly unlikely there are any refurbished iPhone 12s around to buy. Refurbished units are sold by Apple from its own online Apple store only and currently they are not selling them there yet.
Any units provided as replacements during service would have a model number starting with an N, designating a Replacement unit rather than Refurbished.