Altec changed their nomenclature for these cabinets over the years.
The utility cabinets used to follow a naming convention with a suffix letter A, B, C, to denote the size of the speaker mounting hole. The size of the old 612 cabinet matches that listed on the drawing posted by GM, but back in the 60's, that would have been called the 612A for a 15" speaker. But even then, the 612 was the larger of the utility cabinets (6 CF), followed by the 614 (3 CF), and the 618 (2.3 CF).

Confusing, but that is the joy of vintage!