The service element of disability pension shall be assessed on the substantive rank held by the officer on the date of his retirement from service on account of a disability.
Provided that, in the case of an officer of the general list, the service element shall not be assessed on a rank lower than that of a Lieutenant:
Provided further that in a case where that disability arises on or before the 31st May,1963, the service element may be assessed on the paid acting rank held by the officer on any one of the dates specified below which is most favourable to him, namely:-
(a) the date of his retirement from service; or
(b) the date on which he sustained the wound or injury or was first removed from duty on account of disease causing his disablement; or
(c) if he rendered further service and suffered aggravation of the disability during and as a result of such service, the date of the later removal from duty on account of the disability.