Have the gear box out and when I turn the pulley by hand, both wheel cogs are turning as expected, gear box is operating smoothly.
Drive wheels look like replacements.
Under the grime doesnt look like it has done that much for its age.
I will not pull the gearbox apart yet.
I will clean it all up & put it back together & see how it goes.

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Happy is he who penetrates the mystery of things.