That was indeed the morgue & pathology lab. The inside of the fusebox refers to the buildings as "Mortuary Bldg", and the room with the rusted Shampaigne exam table was the cooler room - similar to CVH, there were no standard morgue trays but, rather, one big refrigerated room.

As to Mary Mallon, it's possible that this is the lab that she worked in, but there's no concrete evidence that I've been able to find one way or another.