Two ser#s on the barrels mean the gun was sent back to the factory for the second set, and a new ser# was applied along with the match for the gun. That is why there are fewer Lefever's out there than the ser # range would suggest!
The adjustment screw is tapered, lubricate and let sit, the try and back it out first before trying to tighten on face.

Good luck and take your time,
Craig