This is taken from a NFA sticky over at a machine gun forum:
Any other weapons (AOW's) are a number of things; smooth bore
pistols, any pistol with more than one grip, gadget
type guns (cane gun, pen gun) and shoulder fired weapons with both
rifled and smooth bore barrels between 12" and 18", that must be
manually reloaded .
I am not usually one that is prone to don the tin foil hat but I emailed a guy with gun just like it. it even looks like the same picture but different name I poltely informed him of the situation he never replied and the auction stayed up, setup perhaps?
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