What kind of gearbox is this?

[Fugi93]

First time I seen this. It doesn't look complete, whatever it is.

http://www.gasbike.net/49cc_4stroke_motorparts_gearbox.html#

 

[MotorBicycleRacing]

Gasbike has it mislabeled as a 4 stroke box.
It is a centrifugal clutch that bolts up to the 50 and 66 cc 2 stroke engines.