It appears to be a drum/nipple conversion. Just below and left of the mainspring pin you can see a small circle. This is likely where the frizzen spring screw hole was plugged. There should also be a frizzen spring pin hole plugged just to the right of the other plug but can't see it in the pic. You can also see remnants of the fence sticking above the lockplate just behind the drum/nipple.

Cody

Tact is for those not clever enough to be sarcastic