The very idea that the 'Dallas' canopy "was just an unintended variation made by different batch runs of the canopies" is beyond ridiculous. That they could consistently mould them perfectly in the wrong shape and then go "Aw shucks, what are we going to do with those, oh fit them anyway they'll probably be alright" beggars belief.
If one came out the wrong shape then it would be discarded and someone would find out why it happened and fix it. Even very small changes could have a detrimental affect on aerodynamics and drag so any changes would have to have been approved by NAA engineers first.
Two of the canopies are probably meant to be the 'early' and 'late' Inglewood canopies. If you have problems discerning any difference, join the club, as the there is only a very slight variation in the shape.
The so called 'Dallas' canopy was manufactured by NNA subcontractors. When setting up the Dallas production lines the profile of the canopy for additional subcontractors was altered, very much on purpose, to give the pilot more headroom. It is said that some pilots preferred this canopy. Early model Ks had the Inglewood canopy. Also at Inglewood at one stage a similar canopy to the Dallas type was introduced. Michael O'Leary, author of Building the P-51 Mustang, says that there were at least five variations in the shape of the canopy.
The two types of canopy were interchangeable and can turn up anywhere. There even photographs that show the same Mustang at different times with different canopy types.
There were also P82s (Twin Mustangs) with one of each canopy type. Both types were also fitted to the H model as well.