Does the short go away if you back the centre bolt off ever so slightly? It screws into a nut which I believe is encased in plastic/resin to insulate it from the mag body. If this screw is overtightened I believe it can crack the plastic and short across. I only ask about backing it off slightly as a test as I found I had a similar issue on my mag, although with the locktab folded over it did run fine. I couldn't get my head around what was touching where.
I bought one of those timing lights but off someone else on eBay, a little cheaper than the one from 'the magneto guys' but both would be a worthy investment. Very helpful in my diagnosis work on my bike.