If using fresh clams, wash them in 3 cups water then drain them through a cheesecloth. Reserve the liquid. Cut the hard part of the clams from the soft part. Finely chop the hard part of the clams, the salt pork (or bacon) and onion. If using canned clams, strain the juice. Add water to make 3 cups of liquid. Use this liquid in place of the water measurement given above.
Sauté the salt pork very slowly. Remove and reserve the scraps. Add the minced onions and hard part of the clams to the pan, leaving the pork fat in the pan. Stir and cook slowly for 5 minutes. Sift the flour over them and stir until blended. Heat and stir in the reserved liquid.
Add the potatoes, tomatoes, green pepper, bay leaf and catsup. Cover the pan and simmer the chowder until the potatoes are done, but still firm. Add the pork scraps, the soft part of the clams and the butter. Simmer for 3 minutes and season to taste with salt and pepper.