I've come across the term "back of" (meaning "behind" in a physical or metaphorical sense) in a number of different works from around the turn of the 20th century*. Was this a linguistic fad of some sort that (thankfully) died out somewhere in the Twenties? When used, it seems to convey some kind of rhetorical emphasis which I notice but do not feel, if you know what I mean. It has a sort of slangy feel. Is there a story here?
Also, it often consumes any article that might otherwise precede the next noun. It suppresses a preceding preposition. In back of is written simply back of.
*. I'm an introverted intuitive and often I cannot access my full memories on a topic at first. So I don't presently remember which works they are, but if nobody freaks out about that and changes the mood of the room on me, I may very well remember shortly.