of the rockwell's i have seen previous to 1974, they have had the actual month and year stamped into the face, like 3 69 which would be march of 1969.
what are water head precisions? i did a google search and didn't come up with anything
also a 1inch rockwell with 30 to 40 million gallons, is prob not reading well, i suspect it isnt reading within tolerance at low flow. i was working with a 1 inch rockwell with 8 million gallons friday, it was jerking and sticking at a several gallons a minute flow. you are no doubt loosing money with that 1inch rockwell, i would recomend you change it out.