I have found images of two ovens similar to your DO's bottom part, but besides the fact that the seller does not know who the manufacturer is, they are also advertised at hefty prices - think somewhere between $350 and $380! The #10 is described as "Rare! 1800’s Gate Marked #10 Cast Iron Dutch Oven". The underside sports a single gate mark as well as "H A" in raised letters. The #12 is described in similar fashion and it is double gate marked. This time the raised letters are "C S".