1966 Transmission Identification

[R/T Boy 67]

That number carried on until the end of the 67 model year. By then it was over four million.

 

[mattsmopars]

I thought....maybe incorrectly lol, that the last four digits indicated 7th trans built that day.

That would only be true with the more common 8 digits after the part number. 2 groups of 4 stamping, the first group of 4 numbers would be the 10,000 day calendar date, the second group of 4 being the number built that day.
Matt