I can't say I've ever noticed this one way or another. A quick glance at a few of my games generally shows that black box, non-Rev-A games do not have codes. All other non Rev-A games seem to have codes. See attached pictures of 1942 and 3D Worldrunner. I second the request for pics so we can further investigate.
Maybe someone else more educated on minor variants has some input?
http://imgur.com/a/1y6GmgB...