The answer is in the MAFCA / MARC "Model A Restoration Guidelines", in their Carburetor section. Did you look there ?
Many questions posted here are quickly and easily found within the pages of this excellent reference work. Sadly, it is under-utilized.
Benson,
The upper housing is the first Double Venturi housing made by Zenith; the lower housing does not match the upper housing ,it is a later housing, the boss on the back side is not drilled for retaining the double venture, there is no doubt a boss on the front side that is drilled and a pin installed for retaining the double venture.
Ron
If I'm reading my restoration guidelines right that would be an early A Zenith carburetor made between October of 1927 until February of 1928...
This early top with the Z on the engine side has to be matched with a Zenith bottom with just Zenith on it , no 1, and it is has a domed boss only on the engine side . Very hard to find. I have been looking for one for years.