Maybe the "BBb" notation was conceived independently from the Helmholtz notation scheme
Back in the old "saxhorn" days, it was common to call the small bore baritone a "Bb baritone" while what we call today "euphonium" would be called a "Bb bass". The convention of calling the tuba pitched an octave below the "Bb bass" a "BBb bass" was possibly, simply a way to differentiate between the two horns, merely "borrowing" the convention from Helmholtz, without actually ever meaning to use the label in its original, scientific meaning.
From the 1908 Sears Roebuck catalogue:

The "C tuba" then was called "CC tuba" to differentiate from the French "C bass" (which is in reality a "C euphonium"), just as Rick mentioned earlier in this post (and you thought I wasn't paying attention....
In retrospect, it would have been more correct to avoid "borrowing" terms without actually meaning it (this is where confusion occurs, and why choosing language correctly is critical), and to name the horns in question "C contrabass", or "Bb contrabass".
Unfortunately, I wasn't there at the time, and my time machine is currently not working....






