Hello Larry,
I believe the covers started appearing around the time the factory electronic ignition was used (1969). The originals can be identified by the metal fastener snaps. Later replacements had plastic snaps.
In addition the spark plug wiring had to be routed under the injector lines on cars equipped with the plastic cover. Previous cars without the cover had the spark plug wires routed above the injector lines.
I am not sure if any of the Pagodas had been originally equipped with coil covers like the sedans of the era.
Joe Alexander
Blacklick, Ohio