Perforated steel, with 52% open area.
Most manufactures changed to a flimsy plastic lower cabinet, and backbox vent hole cover in around 1987.(See Photo Gallery)
When I’m doing a restoration, I find most of the plastic covers broken.
These measure 19″ x 3″, or 19″ x 2-1/2″, both will cover the openings.
These can be spray painted, or powder coated.
Prior to spray paint, they should be de-greased.
Some may show very slight surface rust.