IIRC it was the 1964 Trade Structure the changed the ranks a bit. Brilliant Henlow cadets would graduate as Junior Technicians and they had changed the rank identification from the inverted chevron to the four blade propellor.
Very embarrassing the first time I saw one.. A brand new J/T came along. I saw his four bladed propellor and I thought that he was in the Air Training Corps.
"Nice uniform the ATC has got now," quoth I.
Ouch!!!!