The oil filter adaptor plate and fitting has a large round O shaped gasket behind it. The gasket is not normally a leaker and never replaced until such time as an engine rebuild occurs.
On the pre-1957 engines, there was no adaptor plate because there was no spin-on filter. They used a large cannister with a filter element. That exact same gasket came with the filter element and was replaced with each oil change.
The old oil filter elements are still available and include the gaskets. To replace the gasket, remove oil filter and unscrew the adaptor plate fitting with a large socket. Carefully pop-out the adaptor plate using a screwdriver. Be careful not to bend the adaptor plate. Dig out the old gasket and thoroughly clean the surface. Put a light film of bearing grease on the surface prior to installing the new gasket so it will "stick" in the bore while reinstalling the adaptor plate and fitting. Do not use the small round gasket that comes with the oil filter element. That one only works with the cannister type filter. You only need the large round gasket.