That thing is a "vacuum advance" cannister/mechanism on your distributor.

If you don't have one, a "Shop Manual" is a fantastic resource to have to learn about your truck.

Do a web search. They are readily available as new reprints.

~ 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)