It seems like Armour was probably built in sections, so that each new addition almost was its own separate building.  Take for example the Blood Driers, 2nd; Offal Cleaning, 3rd; and Beef Killing, 4th Building.  It seems to have completely fallen off or was demolished from the side of the main slaughter house.  You can see the wall scars of the building clearly on the side of the remaining building.

Update: Demolished in the summer of 2016.