Brand new hubs, so there were no shims at all on lock tab seats before I lapped the joints or before I started shimming...

Are you sure there wasnt a shim stuck to the hub under the locking plate?

I ran into one that had the problem before lapping the ball. Not having a machine shop, I had to hand-lap the dome bottom. Now, THAT is tedious!

I do a lot of hubs and have run into this once in awhile. Taking some off the nut is common when trying to get the best fit possible. I also save all old shims, nuts and parts just in case. I pre fit with the shims on top of loc tab the move under when I make up finial. Steve (CTR)

Thank you for the input. I lapped by hand (no power drill) in a figure eight motion only until I had a matte grey finish on the contact surfaces, but perhaps I could have gone a bit far. Spent an equal amount of time and effort on each of the four ball joints and kept all respective pieces together. All four lock plates mic'ed at .040"-.041", which is quite close for stamped metal bits. My best guess? I may have over lapped... Seemed a bit odd it was only on the LH swivel.

re the locktabs, I got some (chinese made ones?) recently that were part sheared on the corner when press formed. They split when bent up, and were unuseable.
So, eyeball them before fitting..!

It may be that you've lapped too hard, I've had this before.

Also, not all locktabs are the same thickness, sometime you get slightly thicker ones.

Doing the drum to 7.5 disc conversion on my Mk1 van. All parts, including swivel hubs, are brand new. Lapped the ball joints as recommended in this forum's archived posts. Didn't need to deepen the lube grooves in the ball studs, as they weren't greatly diminished by the lapping process. Installed locking plates then started shimming the joints to just the "right feel". Both upper and lower on one hub went just as it says on the tin. On the other hub, joints were beyond loose with the addition of the locking plates. Ended up removing .020" in .005" increments from the base of each cap nut until the pins would lock up, and was able to shim properly from that point. As all four ball joints are genuine Rover, I'm guessing the replacement swivel hub wasn't machined quite to original spec in the areas where lock plates seat. Has anyone else run into this?