Here are a couple of must-have & reputable referral sites PLACED TOGETHER for all US variety collectors to put in their bookmarks.
I just wish I would have discovered these a little earlier in my numismatic career: no telling how many slipped through the cracks. But that's just water over the bridge now:
1) This first link focuses on RPMs only. This one even includes RPM data on
Barber dimes and old numismatic gold pieces.:
http://doubleddie.com/307764.htmlDigging deeper into the above Wexler site will also result in finding RPDs(ceased at end of 20th century) and double dies.
2) This second one below includes both RPMs and double dies but also lists some other lesser known varietal acronyms.
http://varietyvista.com/CONECA%20Ma...Listings.htmI'm fairly certain other lists exist but these are excellent ones to check with first.
Best of luck,
mdpmedia