In large bowl sift together flour and salt. Stir in All-Bran and Bran Flakes. Add raisins and nuts. Set aside.
Mix boiling water and baking soda; set aside to cool.
Cream sugar and butter in a large bowl. Stir in eggs, one at a time. Beat well. Stir in buttermilk and soda/water mixture Stir in bran mixture. Cover tightly. Best if refrigerated 12 hours (or more).
The mixture will keep 4-6 weeks in the refrigerator and can be baked as needed. When ready to bake, drop 1/2 cup batter into large greased muffin tins. Do not stir mixture. Bake at 375 for approximately 20 minutes or until lightly browned. Makes 24 large muffins.