Paul, jumping in a bit, here is some Cirrus Design lore.
Contract numbers are 2 sequences, one for each of SR20 and SR22, of contract positions. Plunk down your money and you get one of these. However, they are sometimes treated more like futures options and are traded between owners. So many such transactions were taking place that Cirrus placed an administrative fee of $500 on them. However, such transactions preserved the contract number.
Serial numbers are the obvious production sequence numbers that you see in the FAA database, one that starts at 1000 for SR20s and one that starts at 1 for SR22s, with a few exceptions that you’ve already noted.
In a perfect world, you would expect contract numbers to match serial numbers.
But, many SR20 position holders of contract numbers upgraded their contracts to SR22s. Cirrus encouraged this by offering pricing incentives. When a position holder switched their order from an SR20 to an SR22, they got a new SR22 contract number and vacated the SR20 contract number. Hence, many SR20 contract numbers were never fulfilled. And I know of a couple of SR22 position holders who deferred their deliveries, perhaps also with contract number changes.
And a few contracts may have been cancelled, as per the terms and conditions of the contract, again causing some contract numbers to not be fulfilled.
Great fun, eh?
And looking forward to your historical observations.