6s1802,
Thanks for the note on the #2925 GT350 details and "Z" stamp. (As you may know it was delivered to Hi-Performance Motors in CA (not by rail).)
Bill,
Thanks for adding #1941 with a "Z" to the list as well. (I see it was delivered to Yadkin Valley Motor Co. in NC. I assume by rail)
I'm curious of your theory if it is different than anything said previously. Send me a PM if you prefer to keep it quiet and want to share.
Just on these two cars alone we can see cars with "Z" stamps were not limited to California dealers or cars delivered by rail. This has been the same case with all the cars I have noted so far. I still need to finish going through the registry to add to the list. (I'm at about 130 "Z" stamped so far just from my VIN photos, with and without the thermactor option.)