1967 Model Year got the regular fine spline, 1966 and earlier was the course spline.
I owned a 1967 Coronet that was all original. It had the 1966 style input shaft much to my surprise. As I mentioned, '67 was a switch over year, so it could be either version.
I doubt it was built that way on the line.
All the part numbers for the converters and transmissions changed for 1967.
Fact. The vehicle assembly plants work off of part numbers.
Fact. The start of a new model year has the plants purging previous model year assemblies from inventory.
In order for you to have a 1967 with a 1966 trans AND converter would mean they didn’t purge inventory of transmissions AND engines.
In 1967 converters were shipped on the back of the engine.
And trust me, if the converter doesn’t engage the trans both go back to the manufacturing plants with demerits.
Only reasonable explanation is the car went to heavy in-plant repair and someone stuck a leftover 1966 trans and converter in it, but it didn’t go down the line like that.
