Perhaps I'm simply unable to grasp what you're saying, Tim, but I'm not seeing what you describe. The coloring (staining, toning, etc) would have nothing to do with (non-existent) serifs though.
Shadows or dirt. Or more likely (although I don't see it on your coin), on clad cents, the copper can be pulled away from the letters leaving the tin susceptible to turning dark (oxidizing).
Follow this link to 1984-d ddr. This shows what split serifs look like on a 1980's Lincoln reverse http://www.coppercoins.com/lincoln/diestate.php?date=1984&die_id=1984d1dr001&die_state=mds