Bedbugbilly,
A competition gunsmith posted:
"Normally the frame and lug are fine. It's usually after someone has "tinkered" with it that it needs attention. So, I've only had to "correct" that joint very few times. Correcting the arbor to a .0025" - .003" bbl / cyl clearance rarely makes for an elevated barrel of any significance. The main thing is having the same revolver every time you assemble it ( driving the wedge in). Correcting the sight picture on that setup gets you on target and keeps you there." --->>>
https://www.thehighroad.org/index.php?threads/diagrams-how-to-adjust-the-colt-lug-joint.873231/
Gauges should be able to determine if surfaces are flat.
I suppose that "if" everything were determined to be squared and properly set up, then it would indicate a problem with the bore alignment relative to the frame...that the bore was not straight.
And that's where the shims can play a role to see how much that the bore is off center.
So maybe it's not the lug per se, but adjusting the lug joint can help to correct bore misalignment.
In addition to using the shims, there's an inexpensive bore sighting laser for handguns that's adjustable from .17 - .78 caliber.
The laser device could possibly help to get a bore to align with a target at various distances with the shims in place.
Then the gun could actually be test fired to see if the sight picture lines up with the POI of the new bore alignment before any permanent adjustments are made to the lug.
635-655nm Windage Elevation Adjustable Red Laser Boresighter Bore Sighter Kit For Hunting .17 To .78 Caliber Hunting Hand Gun
https://www.aliexpress.com/item/328...VBeXICh0v4AGfEAQYBCABEgKmbPD_BwE&gclsrc=aw.ds