According to ATF, it is a machinegun conversion device which are treated the same as a machinegun. Read a little ways down in the link below.Curious about something. Is the possession of this illegal by itself? or is it just illegal if you possess it in conjunction with a glock it can be fitted to?
Internet Arms Trafficking | Bureau of Alcohol, Tobacco, Firearms and Explosives
The Crime and Investigation In 2002, confidential intelligence sources informed ATF Agents of an Argentinean suspect brazenly selling machine guns, silencers and conversion kits over the internet. These firearms were being illegally imported into the United States and other countries. ATF agents...
www.atf.gov