Quoted from site years ago -
"1956 Ford, Mercury experimental dual quad (2x4) setup for 292, 312 CID engines. Documenting these is quite difficult, but what I have been able to piece together over the last 20 or so years is this.
In 1956, Ford ordered 50 aluminum dual quad intake manifolds, casting number ECZ-9424-C from Edelbrock; planning to use the Carter 2361s WCFB carbs used on the 1955 Mercury. Since the Carter has a longer "footprint" than the "haystack Holley", the manifold is cast with the carb mounting flanges further apart than later intakes.
After the first 50 were delivered, Holley came in with a less expensive quote for the haystacks, a.k.a. model H4000, and Ford had different intakes cast for the Holleys, with the mounting flanges closer together. Supposedly, Edelbrock was less than happy about doing all the patterns for a 50 unit order, so removed the Ford casting number, put the Edelbrock name on the casting, and continued to sell the intakes as aftermarket."
http://www.thecarburetorshop.com/Multicarbsetups.htm
If you go to the referred URL. you will see factory release photos of the kit.
Also explains the two differing 8V INTAKES for 1956.