So the pics of the mat I've attached look to be of a reproduction?
First thanks for posting pictures since words do not always get all the info across. First the mat in you pictures appears to have a rubber top - not a vinyl one. If so then its not the cheap repo that has been around for a long time (looks that ways considering how it doesn't bunch and wrinkle in your pictures)
No the backing you have referred to as "felt" could it be jute. I notice the intermixing of a number of red and maybe green fibers on the back side in your last picture with the jack. If so this suggest to me that its an original though the cheap felt ones (which it does not look like) can pick up and hold all kinds of stuff picked up from the trunk floor
Here are a few examples of jute backings. a couple of them are from newer good reproductions while a couple are originals
.......... So the glue on the gas tank was done to keep the cheap repop in place most likely? The filler board looked definately original. Thanks as I almost traded the guy for this mat.
We've seen stuff done to keep the repo mats in place and stuff that appears to have been done at the plant on some cars. Guess determining which is which would come down to looking at the pattern and product used. But this is not something we see on all 68's so far.
For example - this one looks like it had some originally and an owner added to it at some point. I see a courser product and a lighter one that might be from a rattle can of spray adhesive
To contrast look at the heavier application here - IMHO likely an original application. Now could this be two different workers or two different tools for application? Maybe but we may never know - just trying to help make the best choice based on what we have to work with. Plenty more pictures (as usual) of adhesive patterns but they tend to fall into these two camps
Hope this better explains this time