One thing I have found out about these trucks over the years. Just as soon as you are certain that things were made a specific way a specific year... you will find at least one exception. Original exception...
This appears especially true when the vehicles share a year code. Take a look at the VINs reported by members in the past, is there a rhyme or reason to the numbering?
http://6066gmcguy.com/GMC-vin-list-60.html
http://6066gmcguy.com/GMC-vin-list-61.html
There doesn't even seem to be a correlation between the engine number and the year, let alone between the serial number and the year.
And this is just for 1960-61.
With some similarity compare 64-66 which used both a single year code and separate year codes, depending on plant(?)