Cabs of 1937 to '47 Fords & Mercurys often have sliding rear window glass. Initially I assumed these were after-market installations, done at the time the vehicles were restored. However they keep showing up and appear to be identical (locking clasps are the same). Someone stated that these were unique to the western Canada (not Windsor) assembly plant.

Can some clever hobbyist clarify that point. And also, were they standard equipment or optional?

c.d.p.