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All of the tanish colored stuff. It’s fine hair like particles that I can wipe out with my finger.

OP, I can't answer your question, but I can tell you the ProXS does sound great! I managed to get mine installed by a shop in Grapevine, Texas (B&G Mufflers), before a long road trip from North Texas and I love it. Not too loud on cold starts, at least not enough to wake my little girl in the mornings, and not too loud on the road. The thing sends out just enough Hemi noise to let everyone nearby know what's under the hood. BTW, I kept my resos and have made no other mods to the exhaust...for now!
Also, I couldn't help but share a shot of the mailbox attached to B&G's garage. How cool is this?
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