Sort through the beans and discard any off-colored or broken ones. Rinse and drain the beans and set aside.
Remove the rind from half of the salt pork. Cut it in 1/2-inch squares. Cut the other half in 3/4 to 1-inch-thick strips to equal 4 or 5 strips, with the rind attached. Set aside.
Line the bottom of an earthenware crock or bean pot with the 1/2-inch squares of salt pork and onion. Place the beans on top. Bring 1 quart of water to boil in a saucepan. Add the garlic, tomato puree, salt, pepper, dry mustard, maple sugar, molasses, bay leaves and vinegar. Simmer 1 minute. Mix well and pour over the beans. Score strips of salt pork crosswise, about every inch, without cutting through them. This prevents the strips from curling while cooking. Place the strips on top of the beans and liquid.
Heat the oven to 250 degrees.
Cover the beans and bake 5 hours, checking occasionally (first at 2 hours, then every hour), to be sure the liquid is just barely covering the beans. Add more water as needed. After 5 hours, remove the cover and cook 1 hour more. Remove the strips of salt pork and stir the beans before serving.