Stolen from >>>/k/628232

https://www.forgottenweapons.com/submachine-guns/japanese-model-ii-type-a/

>A unique feature of this and other early Japanese SMG designs is the use of an adjustable buffer assembly. As the bolt flies backwards after firing, it is caught by a piston connected to a compressed-air buffer in the rear of the receiver. As the bolt pushes backwards, air in the buffer can only escape through a small valve, which has multiple different sized holes which the shooter can select from. This allows the bolt velocity to be controlled, thus giving the shooter control over the gun’s rate of fire. Our reference book says the rate can be 500 or 600 rpm, but the original report says the buffer has five different holes to select from.