The 904-C is a coupler for a 904-A Low Pass Filter and a 904-B High Pass Filter to make a Band Pass or Band Reject filter.
This module has a very small PCB with a separate edge connector PCB..
The mode switch must be in the off position to use either the 904A Low Pass Filter or the 904B High Pass Filter independently. This scope image show the frequency response of the band pass mode
This scope XY plot shows the band pass response.
This scope image show the frequency response of the band reject mode.
This scope XY plot shows the band reject response.