It is related to the "gate" used by Fisher Body when welding the body panels into a Body Assembly. Think of a fixturing jig.
Posted by John V on the maxperformance Pontiac forum:
>>>It is probable that the 2nd digit represented the fixture and the 1st digit represented the shift (1st or 2nd shift). (edit note, I think I had this backwards, so I changed it)
Similar coding used on the Pontiac Plant Data Plates except that had a lot more "gates" for each Model Series and built the entire line up so something like 60 different "gates" will show up, Fremont had somewhere around 5 IIRC and only built the A body.
Identifying the "gate" helped with QC if it was discovered that there was a fit up problem with the Body Assembly during Final Assembly. Knowing the "gate" allowed them to trace the problem back within Fisher Body.
In '64, this code was embossed on the Data Plate at Fremont with the rest of the codes. For some reason, I think it was stamped on the Plate in '65 presumably after the Data Plate went on the firewall.<<<<
The Max Performance forum was formerly Performance Years before the merger with Ames. Lots of great info that often pertains to our Buicks. Check it out:
http://forums.maxperformanceinc.com/forums/index.php