It sounds as if the gun may have been Re-Jointed using one of the more common techniques suited to a Purdey double under bite, vis new pin or insert in the hook. Works well on a double underbite system but not on a Jones U/L. I would suggest removing the barrels and applying a liberal amount of lamp black to the two locking lugs,particularly the vertical face of each bite. Re-mount the bbls close a few times and check the smear pattern. If the cause is per my hypothesis the forward lump will be wiped clean indicating the Jones U/L is camming the bbls forward. Remove metal from the front lump to provide clearance. Keep us posted on what you find.
