Hi teelau. The ring looks to me like a home modification, never seen such a details on a production model. The scrubbed off serial No's to me look like it's identity was being hidden possibly not for the best of reasons!
Welcome to the forum teelau. It looks like your Model 4 is a variation 6. This was the final version before production ended around 1928/29.
Fourth variation would have seven slide serrations instead of sixteen.oh really? i thought it was a forth variation. What denotes the 6th?