Why? There are S2 Spiders from 1970, both 1750 and 1300 Junior.
You are right of course. Personally I would probably go after '72 as (in the UK) those '70, '71 and build '72 cars go for more as they are road tax exempt. The premium sellers put on for that would take quite a few years to claw back in actually paying the road tax! But that's just my opinion and others might prefer to pay that bit more on the purchase and then not have the annual £165 road tax.