I note that there was some interest a while ago about Balbus. In case there is still interest, I offer the following. From circumstantial evidence, I am 90% sure that the proverbial factoid that Balbus built a wall derives from a Latin exercise book in use in Britain around 1900 ( that is, exercises from Latin to English, vice versa, or both). There must have been some sentence like Balbus murum fecit.

If dxb could remember what book he was using in that stultifying Latin class of his, that would give us a clue.