Stan,

According to a search here gmpartswiki for a "66" master cylinder top gasket, the original GM Part# is 5465109 (actually applies to '60-'66 per the Master Parts catalog).

A search on the web showed this result GM 5465109

This what your looking for???

Hope it's helpful! thumbs_up

~ 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)