The 904-B is a voltage controlled high pass filter. This is a later revision from the three cabinet Moog and the panel has the new logo and module number.
This module has been redesigned to fit on a single PCB.
There are three trimmers in this module - top left: 20 KHz, bottom left: scale, middle right: - output offset.
There are scope images of the high pass filter operation on the three cabinet Moog 904B page.