I think Timken electric two speed rears is correct.
I meant to add that to my reply that the trucks I've seen with that switch
Did have Timken rears.

The 58 task force GMC in my signature has that switch only in 5 speed.
Has Timken rears,

Maybe someone has late fifties GMC parts book that can confirm "Switch knob assembly"
with what rear end option and even a GM part# which greatly helps in searches.

Have a spare in my cabinet.
Tom
Attachments
two speed knob.jpg (12.86 KB, 74 downloads)

Last edited by TGP; 01/21/2021 1:33 AM.
1951 GMC 650 Diesel
1958 GMC 550 Wrecker
1964 Chev C-80 427cid Tandem
1947 Dodge WDX Power Wagon
1977 Chev K-30 400 cid
1984 Chev K-30 Cummins 6BTA