If any casual vacancy occurs in the office of chairman the Commissioners shall as soon as may be after the occurrence of such vacancy choose some other Commissioner to fill the vacancy.

If any casual vacancy occurs in the office of deputy chairman the Commissioners may choose some other Commissioner to fill the vacancy.

So long as they continue as Commissioners every person chosen under this paragraph to act as chairman or deputy chairman shall continue in office as such so long as the person in whose place he has been so chosen would have been entitled to continue in office if the casual vacancy had not occurred.