49 CFR § 179.200-21
Closures for openings
November 18, 2020
(a) All plugs shall be solid, with NPT threads, and shall be of a length which will screw at least 6 threads inside the face of fitting or tank. Plugs, when inserted from the outside of tank heads, shall have the letter “S” at least 3⁄8 inch in size stamped with steel stamp or cast on the outside surface to indicate the plug is solid.
(b) [Reserved]