Jeff, there is no "correct" speaker cabinet. In those days, Sunn had a "mix and match" philosophy. The idea was to buy an amp, then get the speaker cabinet that best suited the sound you were looking for. A player might have used a single 115S, for portability, with perhaps a second 115S for larger gigs. Another person might prefer the 215S. Folded horns were popular in those days, so a 118MH or 215SH might be used instead. Some people might even have chosen a 412S, 612S or 415S.
If that looks like gibberish to you, here's how to decode it. The first digit is the number of drivers in the cabinet. The second two digits are the size of the drivers. The first letter is the type of driver: S for Sunn Transducer, M for Sunn Magna, and V for Cerwin Vega. The H designates a horn loaded cabinet.
Now, all that said, the most common choice was probably the 215S. It also was the same width as the amp, which is not true for all of the cabinets I mentioned.