My impression, based on that lengthy conversation with a 30+ year veteran Leica technician (who actually worked for Leitz) is that Leica hasn't actually changed its mechanism. However, with the economics factor in play, the quality of its parts suffered at different times and more so with recent models.
I was told explicitly to look for used models with sn's in the ranges of:
1974xxx to 1978xxx
1991xxx to 1998xxx
The explanation given was that earlier models (aka "first out of the box") had "bugs", and latter models (those produced after Leica's corporate restructuring/sale) were made with some slightly cheaper parts.
I realize this amounts to heresy in the RF/Leica community, and I was equally shocked to hear it, but like any for-profit establishment, Leica *did* in fact cut corners to save a buck.
As for my own foray into Leica/RF, it has been educational. I'm happy to have had the option to return the M6-TTL and to make a more educated purchase the second time around.