Possible. But the number of digits did not change from 1966 to 1967, so I am not sure why that would be. 1966 format was P and 4 digits (two numbers for month, two numbers for day of the month). 1967 and later was P and 4 digits (number for year, letter for month, two numbers for day of the month).
I was bored so decided to check what I have here at home.
Two M22 cases, one from Camaro and one from Chevelle. Both had normal assembly date format, no funny characters.
Found one case here with the partial 7 and one with the small squared off c - both had normal inspection marks up on the top, center, back edge on the raised area. One had a diamond, the other a diamond with a line inside the diamond. Hmmm....this is the first time I have checked these over that closely.
Maybe the internal tracking mark idea has merit, or just someone wanted their stamp to look "different". I don't know. But there are other instances of inspection marks being part of the assembly date. See attachments, one from a 1964 Muncie and one from a 1965.