27. The days and parts of a day during which an employee, while holding pensionable employment under the Civil Service Superannuation Plan even if, while in that employment, he was a member of the Government and Public Employees Retirement Plan, the Pension Plan of Management Personnel or the Pension Plan of Certain Teachers, was on part-time or full-time leave without pay for a period of not less than 28 consecutive days, which was in progress on 1 July 1983 or which began thereafter, shall be credited, provided the period has not been otherwise credited under this plan, to the employee who applies therefor, if
(1) the leave was authorized by his employer;
(2) he pays the contributions that would have been deducted under the Civil Service Superannuation Plan from the pensionable salary he would have received had he not been on leave according, as the case may be, to the number of days and parts of a day comprised in the leave out of the number of contributory days in the year, namely 200 or 260, according to the basis of remuneration;
(3) he held, in the case of a full-time leave without pay, pensionable employment under this plan, the Civil Service Superannuation Plan, the Teachers Pension Plan, the Pension Plan of Management Personnel or the Government and Public Employees Retirement Plan, even if, in the last two cases, he was a member of the Pension Plan of Certain Teachers from the end of the last leave authorized by the employer or, in the case of a part-time leave without pay, from the end of the authorized leave, unless he became disabled, or, if the leave was followed by a maternity leave, from the end of the leave or, where such is the case, from the end of a leave without pay immediately following a maternity leave.