"C4" suspension code on my fender tag
In 67 there was a Rallye suspension package which included 6 leaf rear springs, HD torsion bars, HD ball joints, and sway bar.
Might be similar in 68?
From 68 salesman's brochure
A FBBO post says C is for suspension 624 is the Rallye suspension and standard on GTS, so C 4 makes sence