Just to add, two weeks ago I purchased another 1965 GMC, VIN I1001PF35005A, 1/2 ton, 230 six cylinder, three on the tree, SWB fender side that was purchased new in May of 1966 (it was titled May 2, 1966); it is equipped identically to my first 1965 GMC. This newest truck had been in the same family since new - until sold to me.
There are no hidden VINs stamped anywhere on the frame - full or partial.
I have also determined - based upon these two 1965 GMC trucks I now own - that the stamped number on the side of the frame below the driver door is at least partially a part number for the frame - number 2443630. I once thought the number immediately following this one was the model year, but this latest acquisition proves that theory incorrect. Below are images of the two stampings - the left is my newest acquisition and it reads: 2443630 11 18 77 67; the right is my first and it reads: 2443630 5 24 77 67. Obviously there are similarities, such as the first seven digits and the last last four digits. Any inputs on the 11 18 and the 5 24 would be appreciated (perhaps month and day?).