The discovery was made by Randy Campbell of ICG, and the coin was then shown to me by Skip Fazzari of that company for my examination. I concurred with the conclusion already reached by Mark Borckardt that the obverse die is new to the hobby.
An observation of my own is that the die state of the reverse is identical to that of BB-255, so these die marriages were clearly struck one right after the other. Since the BB-257 marriage is so rare as to have escaped detection until now, that leads me to conclude that this marriage was the last one used for 1803 dollars. Coins of that date were struck into early 1804, until a presidential order terminated their production, along with that of gold eagles. It's very likely that coining with the BB-257 marriage had only just begun when the halt order was received. How many were actually produced with this previously unknown obverse is anyone's guess, but the number had to be very small.