What is the US Navy rate and rating CS1 used during the late 1940s and 1950s and what would it look like?

I believe that the CS stands for Culinary Specialist. You're on the right track, Aggie, he's a cook but back then the rating was 'Commissaryman' and the rank would be First Class Petty Officer. I can't seem to find a picture of the badge, but there would have been an eagle above the insignia with three chevrons below.
There are five that still exist today. There IS NO evidence that one came before the other, however I would lean toward QM's.

Boatswain's Mates, Quartermaster, Gunner's Mate, Master-at-Arms, Cook (now Culinary Specialists [CS]).

This is just one resource and is the ONLY OFFICIAL site:

http://www.history.navy.mil/faqs/faq78-1.htm#anchor88073