It looks like a garden variety 3 speed overdrive to me. 58-65 units had a rear mount pad so it appears to be the correct for the year. 55-1 trucks had an open driveshaft so they may have offered the OD as well. Even if it isn't stock it's good to have.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine