The datecode info can be found in one of several different locations. The date code tag like Mike showed was found on some Bendix disc brake master cylinders up into some time in 1969. Some master cylinders from the '70s had a bale wire tag too, but on those, the tag had the last 4 numbers of the Chrysler part number, "PWR" if it was a power brake master cylinder, and another couple numbers that I am not sure what they were but don't seem to correspond to any date code.
The datecodes starting in sometime in 1969 were metal stamped into the casting for all master cylinders. I think they were stamped on some pre-69 ones too, like the drum brake with bolted on cap style.
The date code stamp was usually on the side of the casting that was closest to the driver side inner fender. But as Chris said, it sometimes is on the nose of the casting. The date code is 4 numbers, day of the year and one number for the year. 0 123 would be the 123rd day of 1970 (or 1980) though on some with the date code on the nose its reversed, like 123 4 meaning 123rd day of '74.
Many NOS master cylinders have stickers, stencils or even are stamped "made in Canada". I suspect the differences in date codes were different plants. Most were probably made in Canada, but some of the high volume ones like the '67-70 bolt on cap drum brake and the disc brake master used on most '71-'75 Mopars were made both in Canada and some other Bendix plants, and the different plants had somewhat different procedures of putting production marks on their product.
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i did not fine any date codes on my 1971 disc brake master cylinder
