What is this?

Ditto on the seat belt interlock. The little brown "box" you show is the EGR delay timer like Randy said. Here is what my Haynes Manual says:

"Some models are equipped with an EGR delay system that uses an electrical timer mounted on the firewall in the engine compartment which controls an engine-mounted solenoid. The timer prevents any exhaust gas recirculation for at least 35 seconds after the engine has started."

When I installed a Super Six system on my son's slant six, I removed all the EGR components (EGR valve, vacuum amplifier, CCEGR, EGR delay solenoid, etc.). It doesn't need the little box.

Jerry