It's that cad Canadair who thinks it's ugly! Egad...
Having read the relevant chapters of the Tiger Moth Story, apparently the Tiger passed its trials at Martlesham easily and its spinning characteristics were considered "well mannered". During the war, however, a number of aircraft were exhibiting a reluctance to recover form spins and some of the worst offenders were thoroughly tested at Boscombe Down (?). The worst once took 13 turns to recover! Yikes!
Apparently it was the only one with bomb racks fitted. It turned out that the reason was an increase in weight - three coats of paint as the training colours/camoflague schemes were amended, the addition of mass balance weights in the ailerons, plus bomb racks on certain aircraft... The extra weight in the wings acted like a fly-wheel... An impressed Tiger that had avoided the weight gain was also tested and confirmed the findings. The cure was to remove the aileron mass balances and to fit the anti-spin strakes mentioned in my post above.
Looking at the various pics in the book, post war some civillianised Tigers had the strakes removed (or hadn't been fitted with them) others didn't. Example is the famous Sea Tiger, G-AIVW, which certainly still sported them in the early sixties. I sohuld imagine that many of the aircraft that passed through Rollasons at Croydon were recovered in the course of their civillianisation, thus removing the problem.
Presumably the safety-conscious Dutch authorities weren't satisfied with the strake mod and added the ugly "Fokker" fin, though my limited aerodynamic knowledge would have thought a larger rudder more useful for counteracting the rotation! Birch and Bramson considered the mod ugly as well, and were of the opinion that neither it nor the strakes did do much to counteract the problem! I notice 'UDZ doesn't have strakes.