The scratch marks on that piece don't look right, either.
If you want, I can put you in contact with a former shuttle guy who lives in...what the heck is it...Twain Harte, CA (or something) if you live near there.
Nobody has ever found the pieces of STS-107 which were seen by astronomers to fall off over California. I hope somebody, someday, finds them.
It's definitely not a plastic coating, I tried to scratch it with a DVM probe point.. no scratch but metal was taken off the metal probe onto the sample, so at least the surface is quite hard. I read the coating is supposed to be 16 to 18 mils thick, which is roughly 2/100 inch.. looks about right to me (the sample is only 1 inch long, the coating looks thicker because it's always cracked at an angle).
Interesting note, I read that the HRSI coating is a form of silicon carbide, *that* of course is very hard stuff. I don't think this fragment is TUFI, but I would like to know some of the characteristics of earlier tile used on the shuttle.
I'm about 30 miles from your former shuttle guy, it might be interesting to talk to him, but if the sample is far too heavy then it's probably nothing.