M6 viewfinder line thickness

When did Leitz change the thickness of the viewfinder frame lines in

the M? I have an early M6 Wetzlar serial #166xxxx and the frame lines

are thicker, simular to those found in the M4-2 or M4-P. I actually

prefer the thicker lines over the thinner ones found in the newer

leicas. But I have yet to fine another Wetzlar M6 that has them.

I've got a Wetzlar M6 ser# 167-something (I think, it's not with me at the moment)and a very late one, ser# 242-something. The framelines are identical. I had an M4-P and its framelines were identical to the M6. I've never seen any M6s with different framelines. I have an M4 and the framelines, especially the 50, cover a bit more area, and the 90 and 135 framelines are a little different configuration but the lines are the same thickness as the M6. Only my M3 has thicker framelines.

My friend has an M4 and the frame line thickness is identical to my M6 for the exception of the 90mm and 135mm, the configuation is slightly different. I examined an M6 serial #1668xxx, very close to mine in production date, and the lines were thinner, comparable to my M6 .85. I'm beginning to think that the camera may have been modified at one point in it's life.