The last answer was the closest to being correct. The Wetzlar M6 is indeed clearly engraved as such on the top plate, near the back edge above the eyepiece. If it has never been serviced by Leica, it may also very well have a "Leitz" in the red logo. If it has been serviced, it will have a Leica dot. If it's very early, it may also have no strap rub guards and will have a metal eyepiece rim.

The Wetzlar M6 had a different meter circuit, which had a shorter cut-off time and both red lights go out if the lenscap is left on. At some point (actually past the Wetzlar-engraved ones) the cut-off time was increased and one triangle made to blink as a warning if the cap was left on. According to Leica, Kinderman, and DAG it is not true that those early meters are failing more than the later ones. And it is definitely not true that the later ones are 2 stops more sensitive. That change occured with the M6TTL.

There were changes made throughout the M6 Classic, generally attributed to cost-cutting. The most notorious is the plastic film counter. According to DAG, at some point Leica started putting in a reverse-threaded screw in the plastic counter which solves the resetting problem, and this is retrofitted to those which fail. Once done they are as reliable as the older type. IMO not all changes were for the worse. The early M6s have felt light traps around the shutter opening which can be more problematic than the folded mylar strips in later ones. Strap lugs of early M6s are set with a single rivet, which if the lug is twisted can come loose. Later ones are attached with 2 screws from inside, which prevents rotation. Other changes were mostly cosmetic.

I have 2 M6 Classics. One is a Wetzlar, which came to me already overhauled by Leica. It has the later meter circuit, a Leica logo, and a new body covering (without the "Made In Germany" emboss). My other one is a very late (2,4xx,xxx) model, which some silly guy had blacked out all the white engravings on the front, and I had to remove the black marker and repaint the white engravings. Both cameras work flawlessly and each cost me less than $900 within the last year because of slight cosmetic imperfections. Finding the second one prompted me to sell my MP, which had begun to brass and which I had to seal up the eyepiece because there was a huge gap under it. I ended up with 2 M6 bodies and about $200 in change from the MP sale. And even if these M6s ever need a DAG overhaul, I could still break even if I sold them. And I don't have to concern myself with getting a few scratches on them because someone else already did it for me : )