For those that want to be precise:
The last ad of the Ambassador in the Saturday Evening Post (SEP) is in September 1929. The next month, a new Ambassador is advertised in the SEP. The old Ambassador model is re-batched as Envoy in an ad of the SEP in December. But with the new Ambassador coming into play in October, it can be assumed that the renaming of the old Ambassador to Envoy must have happened in October as well. Indeed a newspaper ad can be found of 25 October naming this model now the Envoy. The serial numbers for 1929 range from 900 until 953 (first 3 digits). If sales is taken in equal parts (12 months divided by 53), the 939 serial number ofthe subject watch places this model end of August, and therefore is still sold as an Ambassador.