A 1940 truck with original engine would have come with a Carter carburetor.
Rochester produced their first carburetor in 1949 (a two-barrel).
Later Chevrolet 235 engines would have come with a Rochester type B (yours is a type B).
The cast (raised) number you have pictured is an internal Rochester number for a "blank" bowl, prior to the bowl being machined. The bowl could be machined a number of different ways, and used on a number of different carburetors.
A very few (my guess would be maybe 1 in 20) Rochester B carbs do have the identification number stamped in the throttle body (Gord&Fran post above). Generally the number on the throttle body is also for a casting blank. Only way to know is to find the number and look it up.
There are some basic generic (gaskets and fuel valve) kits that may fit many of the Rochester B carbs. Purchase of the better kits is going to require the tag number.
Jon.
