Sean,
As promised I have had a look at a few french sites to obtain more info about your mustard pot.
Grey Poupon mustard has indeed been produced in Dijon since 1777, and has been sold in ceramic pots from it's earliest beginnings to at least the 1940's. It is still sold today and is made by the French company Maille.
Early pots were quite crude and simple, normally blue or dark blue on white, usually painted by hand, they were produced for many companies and looked very similar except for the company name.
As advertising became more important towards the end of the 19th century companies started to add company logos and designs, these were normally printed and again were usually blue and white.
Written descriptions for early pots normally say they were hand painted faïence, this is possibly why some dealers on ebay etc have listed the pots photographed above as 19th century. However the general consensus seems to be that they are from the 1920's through to the early 1940's. This makes sense if we look at the quality of the material, the form and most importantly the colours used. Think of other faïence produced during this period, from Clarice Cliff in England or Quimper or Limoges in France, the colour palette and shape is consistent with this period.
These pots are not common and are collectible and range in price from 10€ to 35€ depending on condition and design.
There is no definitive documentation to confirm or disprove my theory, however, as the company still exists they may be able to provide more accurate information from their own records.
Regards William.
Ps pots are not mine, just examples taken of internet for illustration purposes.