2351S according to the old Chevy tech info http://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html
There were two carbs used on '55 power pack Chevies---2351S is the second design, and 2218S is the first design with a rather complicated looking throttle linkage, similar to what the Olds V8s of the same era used.