Ok, the only thing I could come up with that makes sense is that the tailwheel maybe wasn't perfectly straight. If it were cocked to the right, that would shift more of the overall weight to the L main. Would that be sufficient to cause the 20 pounds difference?
A quick calculation on the center of gravity indicates that the lateral CG is about an inch left of center. If I assume that the TW was off-center by 4 inches (probably at or beyond the limit of reasonability), the calculations show that to end up with the same CG, the weights can only change by a couple of pounds. Therefore, I conclude that having the TW cocked cannot reasonably account for the difference in weights that I measured.
So, after an hour of figuring on this, I still don't know why the difference in weights, other than 1) some sort of unknown weight on the L wing (fuel? but I'm pretty sure I drained the tanks completely - nothing was drooling on the hangar floor when I got done); or 2) a scale error. It would require about two gallons of fuel in the L tank to cause the problem I'm observing.
And I'm pretty sure there is no fubarite in the wingtip.....
greg