That doesn't seem very likely to me. It would be very awkward logistically (you don't want the frame builder searching for the correct dropout, but to just grab one from the box). And for someone to stamp a number that crooked on a single dropout they'd have to be pretty drunk. To stamp the numbers on a completed frame is more difficult, and the results are more likely to look like what we are witnessing.

Just my thoughts. I am certainly not an experienced number stamper.