Thanks jabs and Kathy L for the prompt identification confirmation! I guess it's a 1950 AA Z variant.
And I was unaware of the watchophilia site. It's another treasure trove of valuable information!
This ambiguity reminds me a bit of Discogs, where they try to catalog music. Small -- often minute -- variations create multiple sub-entries. So when you want to add within your own CD collection catalog Enya's "The Celts," you need to select amongst dozens and dozens of CDs with that title but with different artwork, dates, liner notes, production data, etc. Understanding suffers because of completeness.
I suppose it's a real head-scratcher when some jeweller pops a movement into an unrelated case just for expediency! I'm glad that didn't happen here.