I have a 21-jewel Bulova Phantom like the lower movement shown. This is apparently a later series than the 18-jewel model. Mine is serial number 1306 and has a triangle stamped on the barrel bridge, which I believe indicates a production date of 1926. I know that this symbol can also indicate 1935 or 1945 but I have reasons to believe it is the earlier date. The number "20" is inside the triangle and I am not sure what this means. The movement was cased in the U.S. by Bulova in an 18k white gold case made by Perry Marks. Based on my own research and the case serial number I believe the case is from around 1928.
I wrote to Bulova about my watch back in 1987 and they told me that the movement was made for them by Piguet and was a Piguet calibre 17/22. I understand that there were several Piguet shops in the Vallee de Joux at that time. I have photos I can post if anyone is interested. I also look forward to other information I can learn from this community. Is there any estimate on production quantities? I believe I have seen more of the 18-jewel model.
In reply to I have a 21-jewel Bulova by JerryT