30 CFR § 7.509
(a) Each approved refuge alternative or component shall be
identified by a legible, permanent approval marking that is securely
and conspicuously attached to the component or its container.
(b) The approval marking shall be inscribed with the component's
MSHA approval number and any additional markings required by the
(c) The refuge alternative structure shall provide a conspicuous
means for indicating an out-of-service status, including the reason it
is out of service.
(d) The airlock shall be conspicuously marked with the recommended
maximum number of persons that can use it at one time.