For you guys with a Ford based MG, Speedway has a lot of circle race related parts made especially for the Pinto/Mustang II with a 2300 4 cylinder. Link is below. If you want a catalog, ask for their Race Catalog, which has a lot of Pony Stock pages.
The race Holleys shown in Speedway catalog are the primaries from their four-barrel design. Both butterflies open at same time and it has no choke mechanism. This carb won’t idle until it warms up. My stock 2300 was progressive, with one primary and one secondary butterfly.