Nice work as usual CJO
Mike Rowe is correct, the early ones eventually crack like this. Combination of a loose gun and the cocking slide and extension on the fore-end iron that pushes the slide slamming back and forward. Letting the locks off with the barrels off does not help at all too. I guess Dickson figured this out and moved to the shorter tongue trigger-plate to reinforce this area.
The gun you have there for repair is about as early as it gets when it comes to round-actions. The serial number you provide puts it within the first 28 round-actions made.
The first round-action was No. 3658, built for W.S. Davidson in January 1881 with "Gauge 12 Locks on Trigger Plate". The year earlier Dickson had the patent for the cocking slide which would eventually appear in the round-actions to help cock the locks.
