30 CFR § 18.14
Identification of tested noncertified explosion-proof enclosures.
An enclosure that meets all applicable requirements of this part, but has not
been certified by MSHA, shall be identified by a permanent marking on it in a
conspicuous location. The design of such marking shall consist of capital
letters USMSHA not less than 1/4 inch in height, enclosed in a circle not
less than 1 inch in diameter.
[33 FR 4660, Mar. 19, 1968, as amended at 43 FR 12314, Mar. 24, 1978]