All of the provisions of §§ 207.252 and 207.252a of this chapter, relating to mortgage insurance premiums, apply to mortgages insured under this subpart that provide for interest at the market rate prescribed in § 221.518(a) except that as to mortgages insured under this subpart pursuant to section 238(c) of the Act all mortgage insurance premiums due in accordance with §§ 207.252 and 207.252a shall be calculated on the basis of one percent. The provisions of § 207.252. shall not apply to:

(a) Mortgages that provide for interest during the construction period at the market rate and for interest subsequent to final endorsement at the below market rate prescribed in § 221.518(b); or

(b) Mortgages encumbering a project in which all units are covered by an annual contributions contract issued pursuant to section 10(c) of the Housing Act of 1937.