53. The amortized cost to a taxpayer of any obligation on 1 January 1972, for the purposes of this chapter is:(a) the principal amount, if its actual cost to the taxpayer is not less than 95% and is less than 100% of its principal amount and if the obligation was issued before 8 November 1969;
(b) the actual cost to him if that cost is not less than 100% and is less than 105% of its principal amount; and
(c) in other cases, the actual cost to him plus that portion of the excess of its principal amount over its actual cost determined without regard to section 68, or less that portion of the excess of that actual cost so determined over that principal amount, that the proportion of the number of full months in the period commencing with the day or last time the taxpayer acquired the obligation and ending on valuation day is of the number of full months in the period commencing with the day of such acquisition and ending on the day of maturity of the obligation.