The main reason for the cutoff date was because it was easy to tell a new car from an old car in the 50s. By the time NSRA came along they were all old cars and the difference was not so clearly defined. Possibly some of the NSRA guys had 49s and didn't want to be excluded. Maybe because the first 49s were built in 48 they felt they were included. Maybe they based it on the decade and not the cars.The 50 and 51 Fords were the same car as the 49 and chevys were hardly changed til 53 so the pre1950 thing doesn't make a lot of sense to me.