Here's a direct quote from AMO Specifications
AMO Bow Length Standard is designated to be three inches longer than AMO Bow String Master that braces the bow at proper String or Brace Height. Bow String Master will carry only the bow length designation. Example: A Bow String Master designated as AMO 66" (bow length) will have an actual lengthh under tension of 63".
....omitting some stuff dealing with compounds here....
The AMO prefix to bow lengths in inches means that the bow has been manufactured to a length that properly uses a bow string designated with the identical AMO marking, (i.e. A bow marked "AMO 60", 50 lb will brace to the proper string height with a string marked "AMO 60", 45 lb to 55 lb.)
That's all it is. It's telling you the bow will take a string that's made to AMO standards and marked AMO 52". (Or a string that's 49" actual length.)