Hi Dolores, here is a recipe from the Companys Coming Bread cook book. It is not a yeast bread. I have never seen a yeast boston brown bread. I would be interested in the recipe too if there is a yeast version of the recipe.

corn meal 2 cups.
all-purpose flour 1 1/2 cups
salt 1 tsp.
buttermilk 1 cup
mild molasses 1 cup
baking powder 1 tsp.
hot water 2 tsp.
butermilk 1 cup

Measure cornmeal, flour and salt into large bowl. Stir
Add first amount of buttermilk and molasses. Stir well.
Stir in remaining buttermilk. Turn into 2 grease 28 oz. cans or 3 greased 19 oz cans, or 1 greased 2 quart casserole dish. Fill 2/3 full. cover with foil. Secure foil with string. Place cans or dish in steamer with boiling water half way up sides of container. Cover steamer. Return water to a boil. Steam for 2 hrs. Add more boiling water as needed to keep level up. Remove cans from water. let stand 5 minutes before removing bread from cans. Serve sliced, either warm or cold. Makes 2 or 3 cans or 2 quart casserole. Enjoy. Great with baked beans.