An RPM is an RPM, this one is listed on Coneca: RPM#1, Cherrypickers: FS-01-1968D-501(027.3) Coppercoins:1968D-1MM-001 Crawford: CRPM-001 Top 100 RPM Wexler & Coneca.
I know it's an RPM, I happen to own one. I assumed he was thinking he thought it was a doubled die, aka DD. Only he can answer that question.