Gabriel, Gandy had these comments in the pages, not sure if you read them:
"About 1996/97 plastic parts replaced metal parts in the frame counter, resulting in increased breakdowns. Unfortunately, Leica has not revealed the exact point of these changes. Apparently the problem has since been corrected."
-and-
"Frame Counter has light edge to round dial, shinier silver finish, larger arrow indicator. Could this indicate the change from metal to plastic frame counter parts?"
S. Krauter also confirms the switch to plastic parts in the late production M6, and so she recommends earlier models. In fact, based on the first three serial numbers, she can predict the specific problems your camera will have b/c of her experience dealing with M6 repairs.
And of course plastic is fine if it suits the purpose of the object--but,
A plastic mechanical watch?
A plastic jack hammer?
-I don't think so.
Plastic parts in the frame counter of a Leica?
-I really have no idea; that's why I started this thread.