Russ - Art's black TR3A under restoration has a 5-speed Toyota gearbox which is somewhat like having an overdrive after 4th gear. When I bought my TR3A new in 1958, I bought it with an overdrive which operates in 2nd, in 3rd and also after 4th gear.
The photo shows an "A" type overdrive, but it's not from my TR3A.
Peter in Maine also has a 5-speed Totota. They bought them from Herman in California. Google HVDA