For '56 2-barrel cars:
Buick used only one air cleaner assembly, an oil bath heavy duty type part no 1551770.
Pontiac used three air cleaner assemblies, a non-oil bath standard duty no 1551977, an oil bath heavy duty for cars w/o A/C no 1551978, and an oil bath heavy duty for cars w/ A/C no 1552388.
These pieces are the same on all the oil bath types: element and cover, hold down bolt, internal gasket. So there must some differences in the cleaner base. Buick used Carter or Stromberg carbs. Pontiac used Rochester or Carter carbs. If the carb throat diameters vary, some cleaners may not interchange. For what it's worth, one ad for rebuilt cleaners gives the Rochester diameter as 3".
Sorry the answer is not more definitive.
What has been, can be again. (Bob Wills, 1942)