The serial number is indeed the indicator Jeffrey, I have confirmed this in recent months.

It does hold significance to the mustang range, it was the last mustang to carry the 1970's 'Classic' mustang look, after this machine Victa swapped over to the actual Series 80 engine, with geared side starter.

In the line up for mustang you have:

MKI: 1973-1974 (Series 70 MKIII Engine) [V135]Available in Green or Red
MKII: 1974-1978 (Series 70 MKIV Engine) [V206]Carby is backwards
MKIII: 1978-1980 (Series 70 MKIV Engine) [WA 174 A8] Carb Forwards Marked S80

Cheers
Ty

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