a proof coin can be worn from circulation just like any other coin and since proof is a method of manufacture not a condition a proof coin will always be graded PF-XX. So a PF-54 (if there was a 54, probably meant 55) is a proof coin which shows the wear of an AU-55 coin. These are also referred to as diminished proofs.
I call them "impared proofs". I've found a couple proof Lincolns in circulation, one a PF50 and another nice one about PF65.
Very similar. Same scale. But once a Proof always a Proof. So a Proof coin that has been circulated lightly might grade PF-55 and one that circulated even more might grade PF-45.
I have one just like it, from the junk silver bin. $5.35, and no visible hairlines/fingerprints/discoloration or nicks!