Anything BUILT before 31-12-1972 is tax exempt. The registration date may be later.
Be aware a lot of Land-Rovers out there are floating about on IDs that do not belong to the vehicle, sadly this seems especially true of lightweights, afterall only a tiny fraction of the production span would be tax exempt. Likewise anything that is heavily modified, such as coil spring conversions etc would not be tax exempt due to SVA regs, although this is dependent upon when the work was done, be wary and make sure you get receipts for when the modifications were done etc from the seller.