The "Mule Car" story for the 68 program is pure fantasy.

If (and it's a BIG if) it was any kind of mule car it would have been for the 67 RO/WO program, and that distinction already belongs to the Judy Lilly car (the only other 67 RL21J7 4 speed car known to exist) as confirmed by Dale Reeker who hand walked it down the production line for Judy (see story on my website). It's possible that 2 were built as mules but I haven't seen any facts or verifiable history to back that up at this point in time.