Having bought my first RP in 1959, I believe the trim was original. I subsequently had a few more. One was a totally original standard car (non de-luxe complete with smokers hatch). None had a cover over the gearbox. However the pictures do suggest that some cars did have a cover. I wonder whether it appeared for a short period only.

Location: Near Cambridge

This is the illustration in Jack's link; it would appear to show the gearbox cover as an integral part of the propshaft cover.



Presumably that would make it quite difficult to fit, so perhaps they were quickly trimmed into two pieces, and then no doubt the gearbox cover would soon be left off altogether!

I made what I thought was a gearbox cover using some spare bits of that sound insulation material used for boats. Approx 1" thick, silver on one side with a heavy sheet of something sandwiched between two layers of foam.

Result?

1) it looked awful
2) it reduced the amount of space in which one could rest one's left foot
3) it didn't make a huge amount of difference.

Threw it away. I have found however that an old pair of oily overalls draped around the area has just as good an effect.