I have an original 1955 T-bird carburetor that has been serviced many times through its life. I am in the process of putting new seals in it and upon disassembly there was not a check ball and retainer within the diaphragm housing assembly and there was not a ball above the distributor check ball retainer. The rebuild instructions stated that the distributor check ball should be omitted for automatic transmissions. Also on page 7, paragraph 3, in section 5 of the Holly 4000 Carburetor Manual (found on your website) it states that the ball check value in the vacuum passage of Some Versions of this carburetor...does the word some, imply 1955 carburetors did not have a ball and the retainer?
All Holley 4000 secondary diaphragms require a 3/16" steel check ball. It needs a tiny special retainer (shaped like a capital "G") to limit its movement, preventing it from "seating" against the bore in the diaphragm lid during decel, hence holding the secondary throttles open.
The distributor check ball is trickier, and it has nothing to do with 1955 vs. 1956. Yes, most people use the "no-check-ball-with-automatic" as the rule of thumb, but it does not always apply. For instance, I just finished one for a 1956 Continental that the Holley book did indeed specify to use a check ball --- and you know that car used an A/T! Since I have the Holley service book from the 1950s and I can (probably) look it up by model number, simply send me all of the numbers from the rear of the carburetor base and I'll let you know what the book advises.
Oh, and while the original ball was red plastic, experience has taught me that an aluminum ball will do just fine as a substitute.