You expertise shines through yet again, sir.
Model 1838 Coehorn, I've seen them referred to as 'Royal' mortars with some frequency as well.
Don't quote me on it, but i seem to remember reading that Ames was responsible for every single one manufactured domestically.
EDIT: curiosity got the better of me, so i did a bit of research - Tredegar is known to have produced 76, about half of which were 24 pounders and the rest 12's.
ANOTHER EDIT:
S.N.O.W produced some late-war too, apparently. That more than explains the iron constructed examples.