G'day Norm
LH threads are only used if there is an engineering purpose.

I now suspect that rear roller sprocket tells another story.

If there is a landroll clutch (two clutches on the mower),
then that would explain a LH thread on the roller.

My guess is that a dog clutch (within the chaincase) acts
on this sprocket. This has been removed (in your image).

When free-rolling (disengaged), a LH thread would be needed
to stop the sprocket from undoing.

These are just thoughts - as I have no parts list for the A-V.
But I do have a hint - from an old eBay sale ...

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