After I saw
@irideiam 's post on the Saturday stamping I went and checked out the serial list for 1964. I was amazed at how many 6 day work weeks there were. Overtime for just the workers making the parts or actually the entire factory assembling bikes ect. isn't known. The serial number dates are the dates that the numbers were stamped on the bike's components, in this case the drop out, and then it was used sometime later to build a frame. These recorded serial dates are not build dates of any kind. If anyone is interested, check out all the 6 day work weeks on the 1964 list.
thecabe.com