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Any info on gear linkages and gear adjustments for 75 beetle will be very welcome as it is difficult to change gears from 3rd to 4th . I realise that a manual is probably the answer but it is hard to find a workshop manual here in SA.

How is it difficult? Stiff, or hard to find the correct lateral position of the shifter where it'll move into the 3rd or 4th position? Most of the time the latter condition is symptomatic of a bad nylon bushing inside the tunnel, which allows the shiftrod too much lateral movement so the adjustment of the shifter itself (by loosening the two bolts at its base and shifting its position) is never just right.
There's also a coupler under an inspection plate below the rear seat cushion which can cause similar symptoms, but the bushing's more often the culprit.

http://www.glenn-ring.com/tech/bushing_replace.htm