Thanks for your observations Sixgun..you have a good eye and I appreciate your enthusiasm for the Ropers we both share. The 3rd on the left also has that feature on the reverse side.

The Camp Perry does have an Ivory base plate and is the only one known in the KST configuration. It also letters to the president of Colt at the time. The other rare one in the bunch is top left. It’s the only one I know of in a Police Positive variation.