30 CFR § 36.5
Letters of certification.
When a component or subassembly meets all of the applicable requirements of
Subparts B and C of this part, and also its normal operation will not be
affected by connection to adjacent components or subassemblies, MSHA will
issue to the applicant, upon his request, a letter of certification informing
him that additional inspection or tests of the component or subassembly will
not be required when it is incorporated without modification in a piece of
completely assembled mobile diesel-powered transportation equipment. The
applicant may cite this letter of certification to another applicant who
seeks approval and certification of his completely assembled mobile
diesel-powered transportation equipment and who desires to incorporate the
component or subassembly in such equipment.