Currently, TAC inspectors are being directed to circle "NO" on all certificates regardless. At least that's what I was told by the TAC inspectors in my area. I would offer that the TAC process currently is, at best, better than nothing, but less than ideal.
For Norm Miller, he would put you on his TIROST list only if 2 or more components (ideally more than 2) matched the ledger entries. (engine, g'box, axle and keys) Under the current system, all you have to do is present a shell that has Jensen mods (which changed from series to series) to get a tac certificate. Michael is correct, cars with missing, or incorrect, identifying elements have been tac'd and given certificates. The white car in Texas has an entirely made up (fictional number) JAL tag... at least it's noted on the certificate as being wrong...
it'd be nice if it was harder to get a certificate, or it meant more... The Healey guys have a process for certification of a genuine Austin Healey 100M (640 factory built examples). It mirrors a lot of Miller's requirements. and adds a bunch more.
This would be beneficial to establish values between a car that has "Tiger elements" from a car with matching numbers, components, and original rivets.
As I said, the current system is better than nothing, but is just a first step for establishment that the ENTIRE car started as a Tiger AND that it's the Tiger that the identity plates say it is. Right now, a prospective buyer still has a lot of decisions to make and research to do to establish if the TAC certificate proffered really establishes a value or not. This is where continuous history and documentation comes into play. All of the Tigers I have ever bought had complete histories, original rivets and mostly matching components. The Mk2 I bought out of Canada had the original rear axle, ignition lock, valve covers with the original foil label, and door locks and the tags were affixed with original screws and rivets.. If it hadn't had, I would have passed no matter how low the asking price.
Currently, I also own an Austin Healey 3000 MK2 tri-carb 2 seater (centershift gearbox) 141 of these were built, 5 were factory rally cars. 130 of these came to the USA. I would never have bought it without knowing the entire history and having the identity confirmed 10 ways from Sunday...