"GMC TRUCK DD" is on one line I think and the actual VIN or serial number is on another line. Chevy's and GMC's had the DD. It means Delivery Date, however they did not use it until 66 and beyond, then the date was on another plate. Probably the plate embossing die was made and that portion not used as planed. There are some 66 Trucks with a total embossed date after DD.
Your "VIN" is a GMC number 1001= 1/2T short bed, P= built in Pontiac MI, F= 64-66 year. 21307= the production number built. I believe they started at 10,000 so the 11,307th built, A= 6000 GVW and less.
Nothing I see about your truck is GMC. Everything I see is Chevy, including Custom pillar trim. GMC did not usually stamp the frame, but you may find a Chevy frame number. If you do, you have a truck that was "doctored". As it is, I would examine the pillar plate rivets to see if they are original. Because obviously something is real sketchy. At any rate you have one of the most valuable bodies. Short bed, Fleet side, Big back window, custom cab, factory A/C vents, steel bed, automatic, Good condition, 64-66. Near holy grail.
