SIVAllan
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First would have to be the Harrington Alpines and then I would think the Series I is more rare than the Series IV.
What spreedsheet are you looking at?
Harrington Alpines are not 'Series' Alpines, yes, no?
Most people appear to think Series 3 is the rarest "series" Alpine because it had the fewest made, but Series I is a good guess as it maybe goes to the key issue: how many cars survive to this day...
But Series I is not correct <s>.
I downloaded the register and sorted it by Series, then did counts and compared them to production data, to figure out rarity.
Such a sort and count assumes the register approximates a probability sample which may or may not be true.
Somewhere in the bowels of the search engines's algorithm is the posting of this startling tidbit- I doubt I can find that dataset today.