You all have peaked my interest, I have this coin while going through a group of Lincolns. So what is it ? I see a shadow "D" your knowledge is helpful thanks.
RPM is RePunched Mint Mark. Where the same mint mark (in this case, it would be a D over a D) is punched on top of the same MM. An OMM (Over Mint Mark) is where a different letter (D/S, S/D, etc.) is involved.
Yes. It is a RPM 1955D-1MM-002 http://www.coppercoins.com/lincoln/diestate.php?date=1955&die_id=1955d1mm002&die_state=lds