The LOA's (Letter of Authorization) were for experimental category aircraft only. The P-38 in question was in Limited category. So where he could have legally flown, say.....Lefty's old P-38 (which I am almost positive was experimental category), he was not legal to fly in the Limited category airplane.
Another way to put it is that as strange as it may sound........If you bought Dago Red or Strega, the highly modified, Experimental category P-51 Mustangs, and got yourself an LOA in one of them (now a whole different thing with the FAA), you could fly it and any other Experimental P-51 you wish. However, you wouldn't be legal to fly most other Mustangs, since they are in the Limited category.
Confused yet? I am trying to help, but am not sure that I have.
Gary