So apparently a few folks know about this already, but I stumbled across a post about it and decided to check it out myself while I was doing the last Live Event.
So if you go to the collapsed tunnel to the right of (above and behind) Lamb of God Church aka "Grace's Chruch" you can find an RPG leaning up against a dilapidated truck. Pretty sure that RPG was intended to be the RPG-7 we have in the game, but someone made a typo LOL and when you pick it up you actually get the classic RPG as seen in the previous Far Cry games.
NOTE: because this seems more like a mistake than an Easter Egg there is no proper name or icon for the RPG in the HUD. It will show on the Weapon Wheel, I believe, as an RPG-7 - but it's icon will be whatever was in the slot previously when you swapped for it.
This is one of the weapons coming with the Vietnam DLC. That DLC will contain a few of the classic "soviet" style weapons from previous Far Cry games for the VC enemies to bring to the party. No idea if these will be available in the open world game, but seeing as how Ubi has been introducing all kinds of new stuff I would definitely expect them to activate these in both the regular campaign shops and the map editor.