I think they are NOT standard pipe plugs, yet are plugs with straight threads and a gasket.

Possibly an auto parts store will have one.
Might need to save your old one, and if it's rounded or chipped on the corners weld them back and dress it square again.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-