Provided that a probationer in a class, category or grade shall not be superseded for promotion to a higher class, category or grade by his junior, if the vacancy in the higher class, category or grade arises within the period specified in the Special Rules for completion of probation in the class, category or grade in which he is probationer and if he has passed the test or tests prescribed for successful completion of probation and is otherwise eligible and suitable for such promotion; but his promotion shall be subject to the condition that he satisfactorily completes the probation in the class, category or grade from which he was promoted within the period prescribed therefore, and for this purpose the period of service put in by him in the higher class, category or grade shall be reckoned towards probation in the class, category or grade from which he was promoted and also in the class, category or grade to which he was promoted: Provided further that if a probationer promoted in pursuance of the above proviso fails to complete his probation in the class, category or grade from which he was promoted within the period prescribed therefore, his probation in the higher class, category or grade shall be terminated and he shall be reverted to the class, category or grade from which he was promoted, and any subsequent promotion of such person to the