Like Ben says, the plastic piece shown in image #3 is part of the factory loom that protects the engine wiring harness.
The routing of the engine wiring harness for these trucks (C/K, 10/20/30 81-87) is standard.
It is basically layed out like a "Y". The 3 legs are joined together by a plastic "T" that is down behind the back of the intake manifold. It normally is resting on the very top of the transmission bell housing. That area of the harness loom is secured by a clamp which is a few inches away - towards the drivers side. It is fastened by the uppermost transmission to engine block bolt (LH side).
What you have in your hand is the actual tee that all the wiring for the engine passes through. The ends of the 3 corrugated loom sections butt up against the tee and are fastened to it by wrap-around plastic clamps.
The side of the tee facing out in the image still has it's clamp in place. The back side looks to have a piece of loom still attached and on the right is the 3rd side which has no clamp or loom.
The tee has two connections for 3/4" loom and one for 1/2". As originally installed, the bigger sides go left & right while the small one kind of faces up.
The solenoid wires & battery power enter/exit on the RH side.
The wiring that goes through the firewall and into the cab & also the feeds to the junction block on the firewall, distributor, coolant/oil pressure sensors, etc. enter/exit on the LH side.
The small outlet faces up and the leads to the alternator, choke heater, AIR system diverter valves, firewall mounted ECM, etc pass through it.
Here are some photos that might explain what I am talking about:
