What’s This Part?? (head system)

[MC1600]

1999 4000 Express here…anyone know what this part is and what’s in it? It’s under the cabin sole, above the aux fuel tank, and plumbed between the master head and y-connection to the vacuum tank. 

Are there duckbills in there or anything I need to clean out occasionally? Gonna replace my duckbills and pump bellow and want to get parts ordered all at once (without unnecessarily taking something disgusting apart!)