It's a Wise Child E-6 Episode aired Feb 7, 1974

Parkin goes from one disaster to another. Asked to clear a mess from the yard he has the officer's mess demolished. However Williams still thinks he is his son and constantly defends him. Rangi gets blood samples from the sergeant major and the gunner,proving that the lad is not Williams' son. Parkin is relieved but the men realise that Williams must continue to believe Parkin is his son to keep him in the concert party and stop them from being posted up country. They therefore tinker with the samples to keep Williams in blissful ignorance of the truth.