I have a 235 engine (3835911) that was cast in late October 1953 (J263) and used in a '54 vehicle. The pad by the distributor reads 006219F54X. The first six digits are the sequence number, the F is Flint, and 54 is the year. But what is the X? I can't find any info on that. I see an X used for 1956 Thriftmasters but this is a '54. Can anyone help?