That is the stock SM318 3 speed transmission with the date code for that year. Very common for cars & trucks of that era. The tail shaft and side cover numbers don't always match as a lot of repairs and interchanges occur over the years.
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