The shift box on your truck is from a 55-2 V-8 truck and all 56-59 trucks. The stock shifter box for 55 both series 6 cyl trucks has 2 straight arms and a bell crank on the bell housing with 2 levers for 2-3 gear selection. Probably the previous owner made the swap.
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