The P-51D-5-NA was the very first production model of the -D line of P-51's, of which 800 were produced. They were all sent to Europe and the Mediterranean. As NAA had multiple plants, one in Inglewood, California, another in Dallas, Texas, and another in Wichita, Kansas, North American had already established a method for identifying which of their aircraft were built where. Mustangs were produced at both the Inglewood plant, with NA in the production name, and at the Dallas plant, with NT in the name.
All P-51D-5-NA prodution lasted without fillets being added, until the very last few, as it wasn't until by then that due to the findings/experiences a fillet was designed onto the existing airframe (not really meant to improve handling, as has been often cited, but rather to increase the strength of the tail and help prevent violent snap rolls that tended to tear the tail off). A technical order was issued (in the military, this is something that has to be adhered to) at the same time that the first fillets were introduced at the factory (only the very last few D-5-NA's remained on the production floor at the time) which required all Mustangs aready in the field, to be fitted with the fillet too - these were provided from the factory, through kits, to the fields of operation. This didn't happen until very late summer of '44.
There were P-51D-5-NT's (once again, these being Dallas-produced = NT) used in the CBI/Pacific, but they weren't the same as P-51D-5-NA's, rather they were produced with the same numerous 100+ changes that were introduced with the beginning of P-51D-10-NA production. So the P-51D-5-NT was essentially a Dallas-produced P-51D-10-NA. Thus, it would be much more correct to take a P-51D-10-NA model and put D-5-NT placards/serial number/scheme on it, than use a D-5-NA model.
