BD - The SM319 is a 3 speed with overdrive. The SM318 is the standard 3 speed non-overdrive transmission. Main "3 speed" portion of the transmission is basically identical, but the addition of the overdrive does entail some differences between the two models.

~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)