I seem to recall several recent conversations about Carter YF2100S carbs. You can do a search here: Searching the Site
to hopefully help in your dilemma.
I do recall that a recent conversation talked about Carter YF964S models (for 216 applications) which has a smaller throttle body base to match the 216 intake. A picture (attached below) shows that 216 version having a "1-630" throttle base. May be what you got was the correct upper body with the YF2100S stamping, but with a 216 base mated up (others have said this has been a common problem usually caused by indiscriminate re-builders).
Hope you are able to get the issue resolved.
Am sure others with more knowledge and possible helpful solutions will chime in too.
EDIT: Kevin beat me to the answer - lol!