.............. except for a single raised line running the length of the tube it looks the same as orig - cardboard- flexible painted black 3/4" dia 3ft in length. .
Originals were not painted black but the outer coating was a black or dark gray paper or cardboard based layer. It often separates/unwinds from the tin foil inner layer after all these years. Maybe your referring to the color of the outer layer of the cardboard
Have some out in the garage - will snap a picture of it. Basically similar coating to the ones used on some of the snorkel to heat shields - example 69 Boss.
When it was still available guys were using NOS tubes from Broncos - think one of those would almost make 2 Mustang length ones
Maybe you can post a picture of it ones you get one