FWIW - The Master Parts Books list the pinion gear u-joint flange as Parts Group 5.545, Flange w/deflector. Earliest open drive '55.1 part number listed was GM# 3708838, which was replaced by GM# 3714791 in '57. The latest number that I could find that fits the '55.1 thru '62 1/2 ton truck pinion shaft is GM# 3741822.
That part number also fits all '55-'62 passenger cars and '55-'59 Corvettes. If you really need to replace it, there are a few used and NOS pieces on ebay, mostly listed as Corvette parts to seek higher prices.
Alternatively, there are many after market companies that offer replacements for the flange, for far less $$$ than the Corvette priced pieces. Just do a web search using "GM# 3741822" and you'll have lots of possibilities to go through.
~ 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)
