Yep...the one at the end of where the extractor sits is the front locking lug. (S&W official name).

Your star / ratchet not wanting to go into place when worked slowly is most likely a bent locator pin underneath the extractor star...they will get bent and / or lost, and once gone, even just one, the gun will slowly work itself out of time. You can straighten it by VERY carefully using the body of a replaceable pin punch (the punch removed so that it is hollow) and bending it back into place. The FBI gunsmiths used to make a special tool that would both drive in and stake those pins at the same time but I never saw one for sale anywhere. The rest of us had to make do with home made prick punches, again VERY carefully used so as not to throw up a burr that would cause the star to stand proud. Pins are still available IIRC from Brownell's.

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