I believe it's the ordering/shipping/inventory tag for the rear seat spring subassembly which was delivered as a "lot" of 10. I have a similar tag I found on a seat frame only it was handwritten and for a quantity of 100. (I also found one on the upper rear seat for my 66 Fastback but I left it there.) Your base number, 66800 and my 63810 (C4ZB-6563810-A2) are not shown in the Ford Car Parts so I have to assume that the service number changed or, more likely, the subassembly was not serviced but a higher assembly (or subassembly) was used for service.
Jim