Where a person who has attained pensionable age is in receipt of or entitled to deserted wife's benefit, a widow's (contributory) pension or old age (contributory) pension or is a person in respect of whom an increase of the latter pension is payable by virtue of section 87(1) and would but for section 139 be qualified for the receipt of an old age (non-contributory) pension -
(a) the person may, notwithstanding anything contained in the said section 139, be paid the old age (non-contributory) pension for any period during which the rate of pension payable would be greater than the rate of deserted wife's benefit, widow's (contributory) pension, old age (contributory) pension or the increase thereof, as the case may be, payable, and
(b) in any case where the old age (non-contributory) pension is so paid, entitlement to deserted wife's benefit, widow's (contributory) pension, old age (contributory) pension or the increase thereof, as the case may be, shall continue but the amount o