Wild Bill,
First off, nice find! It takes a good eye to find these.
Many of the 1960-D RPM's can be very tough to id! I have piles and piles of tilted and rotated ones which just make my head spin when I try to figure them out.
As coop mentioned, I find it best to focus on mint mark location first to eliminate a big bunch of possibilities. Then start to narrow it down from there. I know there must be a ton of unlisted ones because I have gone through every picture on coppercoins several of times on some coins and never found a match. What a daunting project to try and figure them all out!
It is both a blessing and a curse that there are hundreds of 1960-D RPM dies out there!