Although I don't really know the real reason for the changes, it is my guess that it was just easier at the time to punch the pins through, because as of the late 70's they really had not yet come up with an efficient way to do it any other way. One more way, even more obvious, that I should have mentioned to tell the range of years of manufacture of LB7 was the serial number. They were only serialized until 1989. The numeric serial numbers were the earliest versions. The numbers with one alpha digit followed, in order from A to Z, and then they started using double alphabetic characters, starting with AA through AZ. If I ever get my records sorted out here, I can more closely determine the range of years by serial number. The production records will tell me exactly how many were made from one year to the next which will help pinpoint the year of manufacture more closely.
