Hmm, what an interesting piece. Certainly early 1920s based on the 10AI movement, but also suspect the case may not be original.
During this period (1913-1923) ladies watches, or wristlets as they were known, usually came in one of three shapes, round, octaginal or cushioned. They had others but these three were by far the most popular in shape, both plain and engraved case versions, as well as different gold quality levels.
It would be a fairly easy to transfer the movement into a case like this as they were all pretty much standard in size.
This is not to say it's not origional, as anything is possible with Bulova, but my gut tells me it's a marry of case and moevement at somepoint in the last 90 years.
My advise, wear it and enjoy the feeling of something nearly 100 years old ticking on your wrist.
This era is by far my favourate in ladies watches.
1922 Bulova Non-conforming (for now).