Yup, I believe b/w is "backed with", too.

And I think there's a "factor of 2" influence in the record speeds; 33.3 being ABOUT half of 78, and 16 being ABOUT half of 33 . . Don't know why they picked on 78 for starters . . (Of course I might just be extrapolating too far from tape speeds, which do go up/down by factors of 2).

G.