Darn, I'm used to editing my remarks, but can't figure out how to here. I see you've got it figured out and can't understand why you might believe this to have been made by AR since there is clearly a bona fide sponsor mark to match the assay mark.
The mark is obviously just a part of the casting and has not been struck on this piece. Since you don't show the assay marks, perhaps they are also cast.
Yes, interesting discussion about his father and his brother Joel and descendants, too. I wish I could find something more about his longevity in the business. I love new acquisitions.
Okay, a little more research result here. The engraving is called an imposed cipher. The Letters are 'F', 'E' and 'J'. Still working on the order indicated by the shading.
I would guess the last name letter to be "I", since that letter is the tallest of the group (and has classical primary shading). 'FEI' appears the most likely combination.