Isn't 2 cubic feet sort of the classic speaker size? It seems to be pretty common throughout an era.
Ran out of time to post; was going to add that its net Vb is ~1.6 ft^3 net IIRC, so add ~0.15 ft^3 for the driver + any bracing.
So it seems! All I really know is that back then, seemingly everybody wanted me to make the various AR/Advent and another popular brand clones with many wanting them vented in the same size too, so 'sticking a sock in it' [vent] became a popular tuning tweak.