In 1959 Pontiac switched from front to side engine mounts, but since all engines from 1955 to 1959 had reverse flow cooling and the 4 bolt water pump, there wasn't a need to recast the cover. In 1960 they discontinued reverse flow and had to redesign the casting anyway, but didn't eliminate the mounting tabs. They continued the 4 bolt water pump through 1964, but didn't eliminate the tabs for the 8 bolt redesign which we have on our 1967s. I'm not sure, but I think they are still on the 11 bolt covers. Someone must have liked them in engineering I guess......John