Going by the serial number I'd say it's a Victa Pro 460 from November the 7th 1983 .
I've had plenty of Victa 2 stroke mowers that when you turn the fuel tap off when running they do not rev up ,they just eventually cut out but have had lots that do rev up from turning the tap off ,I would think when they rev up the engine is running lean as air is being sucked in through the crank case seals or manifold seals leaking.
Good way to test a mower if you are buying a two stroke ,don't buy the mower if it revs up from turning the tap off when running, although I do agree 2 stroke mowers should not be stopped by turning the tap off.
