What you have is commonly referred to as an SM318. While they were supposedly built at the Saganaw and Muncie facilities they are generally referred to as a Muncie. Anaheim Gear which is the source I purchase parts from here lists them in their parts book as a Muncie. The later full syncro 3 speed (7 bolt side cover) is commonly referred to as a Saganaw.
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