The tags you are thinking of didn't start until 1971. Those being the tags that have the last 4 numbers of the part number and may say PWER etc.
On original disc brake master cylinders on 68s and 69's I've seen, there was a similar tag hanging from the bale wire, that just has a date code. The drum brake MCs had date codes stamped in them, not sure why they put date codes on a tag on disc brake MCs, but I'm sure they had a reason?
There's a picture of one of these date code tags in my master cylinder article in the April 2005 Mopar Action.