Plug style seat mount.
Designed to help mount seats as low as possible to the frame. 
3/16” thick (or the plug lite is 1/8") nickel plated, raw steel or now stainless.
Now available with optional spark plug wrench.
Comes with o rings to fit tight to a stock ID Panhead or knucklehead rigid frame seat post bushing (1.2" ID) 
Seat post bushings also available