These are the BLAU 110mm Aluminum Fuel Caps used by the Porsche factory on the 100 Liter 914-6 GT Rally Fuel Tanks. These large ALL Aluminum fuel caps were originally used in several other Porsche racing cars, including; 906, 908, 907, 910, 917 and 911 RSR https://pbase.com/9146gt/blau_110mm_cap_fuel_aluminum_from_917k. Very, very rare pieces difficult to source these days.

For illustration and comparison purposes, I also included the significantly heavier BLAU STEEL Fuel Caps https://pbase.com/9146gt/blau_110mm_cap_fuel_all_steel_not_period_originals. Those are NOT the lighter Porsche factory racing parts, more common parts that anyone can still easily source and purchase today.

Here is an interesting comparison between the 110mm BLAU All Aluminum and ALL Steel 110mm Fuel Caps. It goes to show you the great length Porsche went through to gain ANY weight savings advantage in their racing cars; https://pbase.com/9146gt/blau_110mm_cap_fuel_alum_vs_steel