You ask when did Forage caps and No 1s stop? You might also ask when did berets and No 2s stop. I have a picture on my uncle in his barathea No 2s and beret.
I'm hazarding a guess I'm younger than your uncle
P-N. Certainly this was still seen at Catterick c 1965 when I was a newly commissioned Rockape. Had to be an officers' beret (usually from Gieves), an airmen's beret with an officers' pattern badge wouldn't do. Also when I was in Aden we wore DMS boots with Fox's officers' puttees with our KD socks and shorts, much to the amusement of just about everyone else who sensibly wore desert boots with theirs.