The fuel / weapon codes were platform specific
Incorrect.
The first letter was platform specific, the fuel/weapon codes were generic. The 50001D listed codes for a list of aircraft including F15, F16 and other common NATO types. These were required for entry into computerised system nt able to use the USAFE code system.
The USAFE codes were in a USAF manual. not a NATO standard. It was just that, as usual, there was no common code, so everyone used it. To the extent that it was still be used 10 years after USAFE was disbanded as a command and no other command adopted it as an orphan document and it stopped being updated.