Written on watch face ~ Bulova Swiss. Written on inside of back cover ~ Pat. June 10, 1924 Bulova 14K Gold Filled New York C274705. Written on watch mechanism ~ 6 Adjustments 21 Jewels Bulova Watch Co. 9AF. Written on watch band ~ Maxim Pat. Pendg. I searched the ad data base and could not find although the 1930 President and Ambassador look close ads were not clear enough to confirm either way.
In reply to Jim - Once you have this one by Geoff Baker
Early watch crystals are mostly made of nitro-cellulose, this material is basically made from wood pulp, with certain additives and chemicals which with age will yellow. I have read that a chemical called lignite is the main cause of turning yellow.( "lignum" is Latin for "wood).
Think of how paper yellows with age, this too is made from wood pulp as you know.