AOW mean Any Other Weapon, under the 1934 National Firearms Act. it's a weapon that falls under the other NFA classifications, like Short barrel Rifle, Destructive Devices, Machine Guns etc... weapons like pistols with forward grips, pen guns fall under that AOW rule.
The stamp, refers to a tax stamp you have to pay in order to build, or transfer NFA weapons. It's actually a physical stamp that is placed on the document that says two hundred dollars on it. Or in the case of a transfer AOW five dollars. But it still cost 200 dollars to make it