Part# KB-NPTF-MS-114 ; 1 1/4" NPTF national pipe taper thread pressure plug.

Same application as used in the Ford motorsport blocks where the welsh plug is tapped 1 1/4 inch pipe thread to accept a screw in pressure plug like these.  The idea is to prevent the welsh plugs poping out in racing applications.

The tread is tappered so as you srew them in it binds against the standard tap thread in the block to form a pressure seal. Usually installed with plumbers thread tape wrapped around the thread of the plug to help it seal.

Make sure you check your sizes required in your engine before you order. They do vary in size.  You may have to retap your block depending on thread type, these are not so easy to source in different thread sizes.