Tom,
There is just no way to know. Some of the information I have goes back to cars that were still around in 1975, some from the SIAC club from 1978-92 and some from Jeffery Coe's original T/A Registry in the '90s. The only way I will know for sure if some of those cars still exist is if the owners actually contact me.
As far as T/A owners that I have the names and addresses for, there are currently 588 in my address book. So, less than half of the T/As I am currently aware of have actually been registered by the owners.
If the others would register, I wonder how many MORE broadcast sheets they would find for other cars after I asked them to look?
Maybe we will know someday.....
There are T/As out there that only a few people may know of, so for those of you that do know T/A owners, please ask them to get in touch with me if they like.
Thanks,
Barry