Spin strakes were not required on Oz Tigers or Chipmunks, never had issues spinning either myself. Some were of the opinion that the Chippy earned an unsavory reputation because of the length of time it took to recover, instead of waiting for the recovery action to take effect folk would become alarmed that recovery was taking too long and then start trying alternative methods - which would not work. The Tiger spin was rather benign I thought and used to enjoy it, anyone with an insight into why strakes were fitted?
They were fitted soon after the type's introduction after a number of accidents. The issue was determined to be the blanking of the rudder by the tailplane at high angle of attack. The strakes create a vortex which flows over the rudder re-energising the airflow to make it more effective. Other types such as the Tiger and Miles Magister had them. At about the same time, a wide chord rudder was fitted. This was not to improved spin recovery, but to improve handling during aerobatics. But somehow the two got included together in the restriction.
The entry to the spin in those days was usually from the stall, whereas when I was instructing on the RAF's Chippys, we entered from a positively entered semi-flick manoeuvre with full rudder shortly followed by full up elevator applied at 50 knots. This led to a very stable nose down spin which recovered quite rapidly. I flew a number of spin entries from the stall (not all intentional!) which led to a slower, flatter spin which took far longer to recover. I suspect this flatter spin prompted the introduction of the strakes.