Gates was/is the GM supplier for the 12576446 A/C Belt and also to AC Delco as P/N 4K365. It fits all four years of the SSR, both 5.3 and 6.0 liter applications.
Part Number Cross-reference:
GM 12576446
AC Delco 4K365
Gates K040365
Please note that your belt may come with either Gates or AC Delco packaging.