Note the 9 and S. The S is similar to 1945s-1mm-006 on the copper coins website. Could this be a very rare error?
Including links in your posts helps. https://www.coppercoins.com/lincoln/diestate.php?date=1945&die_id=1945s1mm006&die_state=mds Seems like the secondary MM should be visible and it's not, so I think it's just beat up.
Anything pertaining to mint marks fall under the Variety category not the mint error category. I don't really know about yours. The 9 is flat. Nothing there or rare. IMHO
The diagnostic for one year is meaningless for another "Ball serif mintmark. An extra top half of a secondary mintmark shows spread half the distance of a mintmark to the south. With die progression, the part extending to the south of the primary mintmark disappears."
It may be the RPM you posted but with the damage to the date and MM it doesn't hold much value. The MM placement looks correct.