Jeff, was this only a Dearborn/San Jose thing? Have you ever seen the screw on a Metuchen-built '67?
Good question. According to the Osborn Assembly Manual, as mentioned earlier, it was handed down to begin to use the screw in the middle of October 66. As to when the individual workers at the different plants GOT the memo or ADHERED to the instructions, it may be hard to determine.
If not sure your example had the screw or not, FIRST, by default, I wouldn't simply ADD one. It might be best to consider using the actual build date for an idea if it could have been used. If built AFTER November 1, 1966...the screw was most likely installed. If built before the 2nd week of October, most likely was NOT installed. If in the 2-3 weeks of "not sure", I would look for other examples closest to your actual build date for guidance. If you have the original doors you could look for a hole drilled in that location too.
One other thought...Just because there was a screw there OR there was a hole there alone isn't probably the best way to decide. I am sure if I have December built example with a DATE CORRECT DOOR and NO HOLE...and there is no sign of ever having the screw, I certainly wouldn't just add one either!