Carl is probably best to chime in on this because he works at a dealership but my experience with thermostats is that at most places, including dealerships, you will find aftermarket producers such as Stant dominate the market. I dont know for certain if GM ever actually manufactured their own. They are available at Canadian Tire, Piston Ring, NAPA and others. Thats where I buy them for my engines. They havent changed in the 45+ years Ive been buying them, except maybe for the holes they have drilled in them to allow a low volume bypass of sorts which really helps to avoid air locks in the system. Never had a problem with them.

I just called the Exeter dealership and he tells me that number is not in the system??