Any company incorporated under this chapter may, subject to the approval of the Secretary of Commerce, borrow funds and secure the repayment thereof by bond and mortgage or by an issue of bonds under a trust indenture. Each mortgage or issue of bonds by a company formed hereunder shall relate only to a single specified project and no other and such bonds shall be secured by mortgages upon all of the real property of which such project consists. The bonds so issued and secured and the mortgage or trust indenture relating thereto may create a first or senior lien and a second or junior lien upon the real property embraced in any project. Such bonds and mortgages may contain such other clauses and provisions as shall be approved by the Secretary of Commerce, including the right to assignment of rents and entry into possession in case of default, but the operation of the housing projects in the event of such entry by mortgagee, trustee or receiver shall be subject to the regulations of the Department of Commerce under this chapter. Provisions for the amortization of the bonded indebtedness of companies incorporated under this chapter shall be subject to the approval of the Secretary of Commerce.