Spelling changes..or doesn't

In the inscription for Ruta, one thing which we have seen before is the tendency to replace "v" with "b":  using "bibet" instead of "vivet" - "she will live".  We see this in Spanish. 

Another curiousity is the fact that we have the adjective "affabilis"  spelled "atfabilis", which means, literally, "can ('abilis') be spoken ('fa') to ('at') ".  We have words derived from the same roots: affable, 'that which can be spoken to', hence, 'easy' to speak to', 'approachable', and ineffable, 'that which cannot be spoken of' .

What is the "at-" prefix, though?  This is a relic, because the "af-" of "fabilis" was originally "ad-", so this word would have been spelled "adfabilis".  "Ad" is familiar as a regular preposition and prefix in Latin meaning 'to' or 'toward', and, as a prefix, frequently changes its spelling to assimilate (ad-similate) to the word it was being attached (ad-tached) to.  So, ad + fxxxx gives us affxxxx.  In this word, however, the 'ad-' prefix, rather than assimiliating to 'af-', merely softened from a 'd' to a 't'.  Its persistence reminds us of its genesis and original spelling.