It's my take that cases with the following case back were 1915 -1919.
I'm still not 100% certain when the Glode was introduced, but it is still my take that it's 1920. The earliest Globe stamped case that has been date engraved 1920 (that I've recorded) had a serial number of 1006919.
Based on how Bulova dated their early watches 11xxxxx 12xxxxx, 13xxxxx, 14xxxxx, 15xxxxx, 5xxxxxx 6xxxxxx, etc, I'm still inclined at this stage to use the KISS principle and say that 10xxxxx stood for 1920. This may not neccessarily mean they 'sold' in 1920, but were manufactuted in 1920.
Hopefully one day we do find some nice evidence (a repair receipt or similar) to really show when the earliest 10xxxxx stamping was used.
Although with all that said I could be 100% wrong :-)
Subject watch for me (at this time) would still be a 1920 Bulova Rubaiyat