21 Jewel 1939 has me a bit puzzled. There were a multitude of watches that looked similar in this time period. Band apears that it could be original and would have been factory equipment on an Ambassador A. I thought at first it might be a Oxford/Montgomery, but the face is slightly different if you look closely at the "0" and "2"s. The Oxford and Montgomery models has a "0" with a slight peak on top and "2's" that were more blocky without the flourish at top and on foot. They were also 17 jewels; this one is a 21 like the Ambassador and Eagle. Lugs appear too smooth to be an American Eagle.
I found a couple ads in the short ad section that are from the same year picturing a watch referred to only as a "Bulova 21". Ads seem to indicate that this was a model name, although there isn't an example in the database. Ads appear to show a face identical to subject watch.