The book “The Latin Language” by L.R.Palmer (page 127) says that “Or. 158” by Cicero says that “insuavissima praepositio est “af”, quae nunc tantum in accepti tabulis manet ac ne his quidem omnium”. What other information is there about this preposition “af”? What did “af” mean?
Check the definition of “ab” in Lewis & Short. In a nutshell, “af” is a weakened form of “ap”, which in turn is “analogous to the preposition apud”. Cicero considered it archaic.