The amp chassis is obviously a guitar amp mounted upside down. It would be a model with vibrato given the rear panel control for vibrato speed.
As for the controls, it may be possible that concentric controls weren't available at the time or there were so few of this model made that they opted to use existing stock rather than buy a large quantity. It is anybody's guess what the controls do, whether it is a preamp or just extensions of the controls that used to be on the adapted guitar amp.
That's a cool find though!
