I've owned the car since 1976, and never noticed the existence of the stamp marks until 2 or 3 yrs ago, and after reading JohnZ's believable explanation of what they meant, I checked and found what I found. It was more recent than that, after removing my Booster/MC for rebuild that I found a faint '2nd' T mark below the cowl tag.
As far as explanation, I'm not sure what you mean unless its why they aren't on both sides on all cars? I suspect that situation is due to having a single person doing that inspection in the Fisher plant, rather than one on each side of the car.