My grandmother was very economical with words, so my copy of her recipe simply reads:
1 c. graham flour
1 c. cornmeal
1 c. rolled oats
3-4 Tbs baking soda
1 tsp salt
3/4 c. molasses
2 c. sour milk or 1 3/4 c. water
1 c. raisins
Makes just enough for a 3 lb can. Steam in boiling water for 3 hours.
The Joy of Cooking recipe, found on page 773 in my 1997 edition of the cookbook:
This is the moist, dark molasses bread traditionally served with Boston Baked Beans.
Grease two 4- to 5- cup molds, such as pudding molds, small heatproof bowls, or 8 1/2 by 4/12 inch (6-cup) loaf pans. Have ready a steamer, Dutch oven, or kettle with a lid large enough to hold both molds, or two smaller pans with lids to hold 1 mold each.
Whisk together thoroughly in a large bowl:
1 c. yellow cornmeal
1 c. rye flour
1 c. whole-wheat flour
2 tsp baking soda
1 tsp salt
Whisk together in another bowl:
2 c. buttermilk
1 c. chopped raisins
3/4 c. light or dark molasses
Add to the dry ingredients and stir until the batter is well blended. Divide the batter between the molds. If the pudding molds have lids and clips, grease the inside of the lids and secure the clips. Otherwise cover the molds with a double thickness of greased aluminum foil, greased side down, and secure tightly with kitchen string. Set the molds on a trivet or a folded towel in the steamer. Pour boiling water into the steamer until it reaches halfway up the sides of the molds. Cover the steamer and turn the heat to high. (If the cover fits loosely, you can weight it with another pot or pan to keep the steam from escaping.) When the water boils, adjust the heat to that the water simmers gently.
Steam loaf pans for 2 hours and deeper containers for as long as 3 hours. Replenish the water in the steamer as necessary. The bread is done when a toothpick inserted in the center comes out clean. Transfer the containers to a rack, uncover the breads, and let cool for about 20 minutes before unmolding.
Serve warm or let cool completely on the rack before wrapping to store. The loaves can be reheated in a 300 degree (Fahrenheit) oven of slipped back into the molds and resteamed before serving. To slice without crumbling, use a tough string or dental floss and a sawing motion.
I also mentioned in that comment thread that King Arthur Flour Baker's Catalogue would carry a mix for it if anyone did. I checked my latest copy of their print catalog and not only do they carry brown bread mix (item 3249), they have a lab glass bread steamer (item 5160) equivalent to the size of a 1-pound coffee can.