numerical acronym

[Michael Adams]

There really are two classes of word in question here, yes?  "b4" is an example of laconic (word-rebus like) forms; K-9 is a shortening that looks like an initialism (or alphabetism) -- NOT an acronym, but with a number (which isn't an initial).  So the question is, do we recognize a class of shortening that combines initials and numbers, or do we lump the phenomenon into alphabetism/initialism, faute de mieux?

Good question!