[unruhjonny]

I would chalk that up to an error, or possibly a later version where the 6k rpm tach was for some odd reason deemed as the superceded part;
I'd guess the former before the latter.

I strained my eyes as much as I dare against that tiny image;
But to back up my assertion of it being either of those suggested scenarios, that appears to be a somewhat generic parts book from 1987 - over a decade after the 1976 cars were built.

Both of my 1967-1976 Firebird MPC's do not show that part number.
My 1985 printing of the "1977-1981 Pontiac" MPC shows that number as 1977 & 1978, but also gives a different part number for a revised tach used for 1978-1979.

But pulling our heads out from behind books for a moment, there is the overwelming first hand accounts of what people had..
Then there's the accounts of parts people pulled from purported unmolested cars. I have personally been around, pulled parts from, or worked on a good number of 1976 TA's, and they all had 8k rpm tachs.

Please consider adding all of this as well as the previously held general consensus (that the tach rpm limit change being for the 1977 model year) into account.