Just to add to this, there are also some additional numbers stamped, not cast, into both sides of the outside face of the caliper. In the images are NOS left hand and right hand calipers, with both faces shown, along with the date stamped box (which looks like 10th May 1979, albeit the bottom part of the second 9 is missing), for the LH caliper. This does correspond with the "date" number on the caliper face. 

Yes, @Captain Obvious  Date on the door tag for my car is 6/71.  Here are mine:

  

 

 

 

OK, so another data point that lines up. So unless someone else comes up with something better, I'm going with what we got.

@SteveE, It was your pics that turned the corner for me. I had some 70 calipers here that were "0" year and a 78 pair that were "8", but I just couldn't believe it was that simple. Then when you posted your pics of your 8/72 build calipers that were all "2", it seemed that it WAS that simple. Thanks again!   

 

Glad to have helped.  Thanks for your contributions.