It's been my opinion that these extra marks are a manufacturer's date code or something similar.
You're probably right. I didn't see it in the 74 or 72 TSMs (the only ones I checked) but aren't twin-grips marked by double letter stamps? Eg. "HH" is twin-grip, "H" is plain diff?
Not that I'd expect to see a low-ratio amc15 in twin-grip... as much as I'd like one!