Reb, the Coltons came in several models, and the Models 1900 and 1901 were boxlocks with sideplates shaped like bar action sideplates. The Model 1902 is the only one I have seen that did not have the typical bar action shaped sideplate, but it is a sidelock with sort of a similar shaped plate as yours. All of these models cocked with the opening of the thumblever like your gun does. I think Charles and Edward Lefever worked for Colton at least in 1900 and 1901. I have not done it, but a patent search for the Lefevers and Colton might turn up some answers. All of this assuming I "might" be on the right track on your gun.
I agree on the spring loaded extractors/ejectors. Otherwise, why would they go to the trouble if the final result was only an extractor.
