What you have is presumably a Series 3 Stage 1. One source of manuals is to purchase on DVD from the AULRO shop, which helps maintain this site. See the bar at the top of this page.
As far as i know, all Series 3 (including Stage 1s) were negative earth, and I can think of no reason why this would have been changed. The change from positive earth was made in 1967.
Sounds like the previous person got it wrong, which is likely to have had a disastrous effect on the alternator, although I don't think anything else will have been affected (unless it has a non-original electronic ignition module).
More details of your purchase and pictures would be of interest.
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