I read somewhere that it was not uncommon for English button makers to make buttons for the US Govt in hopes of getting a contract. When the deal fell through, they simply put the shank on the design side of the button and the front was now just plain. Yours appears to lack any wording that one typically finds on back marks so maybe that is the case with your button? Perhaps someone will chime in with a better ID. A great find nonetheless, congrats.