5(1)In paragraph 2(d) above " invalidity pension day " means a day—
(a)for which the employee in question was entitled to an invalidity pension or a non-contributory invalidity pension ; or
(b)for which he was not so entitled but which was the last day of the invalidity pension qualifying period.
(2)In sub-paragraph (1)(b) above the " invalidity pension qualifying period " means the period mentioned in section 15(1) of the principal Act or, as the case may be, section 15(2) or 16(2) of the [1975 c. 60.] Social Security Pensions Act 1975 as falling within the period of interruption of employment referred to in that section.