The National Archive at College Park Maryland has the original ledger copies
of the Consular Post Record: Registration of (all) American Citizens (living
in the city of the Consulate). They have them in Record Group RG 84 for
cities all over the world.
My wife and I searched the set for Havana Cuba for the years 1920 - 1935.
New documents were required every 2 years. My grandparents arrived in the US
before 1900 and were naturalized citizens. We found several critical
postings. They contain a wealth of family vital statistics, historical
information and even in some cases original photos attached to the archive.
The record helped fill in some large gaps in my understanding. One post
especially confirmed my speculation about my grandfather's birthplace (a
very small Polish town - previous records always said "Minsk") and another
gave me the complete set of names for his nephew's family. Now I can try to
trace his family.
They are fascinating snapshots!