Proper belt for Goldmund T5

Been using a Goldmund T5 tonearm for a week or so now. Sounds fantastic of course. The drive belt is an o-ring that the previous owner got from a bearing house. Buna N and probably 70 durometer. Does anyone know the spec of the Goldmund o-ring? Viton? Hard or soft durometer? The generic o-ring wants to slip out of the clutch. It seems too soft and slippery. Any help I can get is appreciated Input from dealers/manufacturors is also welcome.