73 340 duster

Zebart was a waxy yellowish stuff that they sprayed mostly from above and inside panels.
Drilled holes and sprayed inside panels with a long wand then a snap in plastic plug covered the holes.
Worked really good in salt country.

I don't remember if they offered true 'undercoating'
if black undercoating covers the floor and overspray on the sides of the fuel tank, exhaust hangers, etc ... it's probably dealer installed or some other aftermarket deal like Rayco.