When appropriate, the Secretary shall issue interpretative bulletins interpreting the standards under the authority of § 3280.9 of this chapter or interpreting the provisions of this part. Issuance of interpretative bulletins shall be treated as rulemaking under this subpart C unless the Secretary deems such treatment not to be in the public interest and the interpretation is not otherwise required to be treated as rulemaking. All interpretative bulletins shall be indexed and made available to the public at the Manufactured Housing Standards Division and a copy of the index shall be published periodically in the Federal Register.