The Bell Burger was a hamburger bun based meal on Taco Bell's menu from the mid-1960s to the mid-1970s. Created to combat the hamburger, the Bell Burger was a bun with taco meat, grated cheese and mild border sauce between the buns. The Bell Burger would be superseded by the Bell Beefer. The meat was seasoned, there was no sauce! You added that yourself!
The Bell Burger was not superseded by the Bell Beefer, it was renamed. Then of course dropped from the menu prior to the "Think Outside the Bun" ad campaign.