The shop manual doesn't really display the whole thing broken down, but here's an exploded diagram of the SM420 from a 1929-57 Chevy Master Parts Book.
The "3rd and 4th gear clutch" is Parts Group 4.380. It works together on the main shaft with the synchronizers, synchronizer hub, keys and snap ring (all Group 4.384) for shifting the gears between 3rd and 4th.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "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)
