Found these in the garage attic where some of my '68 FB (SJ) parts are stored. There are a couple of minor pieces from a '78 Mach I stored in the same spot. I'm pretty sure nothing like this was used on '68 Mustangs, and the '78 has not been apart since my folks have owned it (the pieces mentioned earlier are the trunk carpet and an NOS heater tube).
I did part out a '69 Torino GT fastback 30 years ago. Seems like the only possible source for these. If so, it looks like they would go around the rear wheel housings. Can anyone confirm?
Interesting to note, they are the same materials as SJ Mustang firewall pads. If someone can identify the source, other similar vehicles may then become a source for "graft" material to repair firewall pads.