The 15" wheels were included with disk brakes on the full-sized cars only. The Chevelles, Chevy II/Nova,and Camaro still used 14" wheels as standard with disk brakes. BTW, Rally wheels were standard with and exclusive to disk brake cars in '67. For '68 Rally wheels were still standard with disks, but were now optional on drum brake cars as well. For '69 (and possibly some late '68 models) the Rally wheels were no longer standard on disk brake cars.
15" Rally wheels debuted on Chevelles in '69 with some Yenko SC (COPO). They didn't return in that size on Chevelles again until the '74 Laguna type S3 and/or radial tire option.