JJ, that isn't correct for the '48. It'll probably work - with a lot of work - but the original has a boatload of screw holes in it/them. The windlace for the '47-'48 is different from later years. Like you said, they're just flat pieces with raised and rounded edges.
If you happen to come across a set, here's a hint for you. Install the windlaces before you install the and the glovebox. Why? Well, if you look at either end of your dash, on the sides facing the door jamb, you'll notice a small hole. This hole is there to provide access to one screw hole of the windlace. you put the screw on the screwdriver tip and, from inside the dash, put it through the hole and into the corresponding windlace screw hole.
Maybe you can get those two screws in withoug going from inside the dash, but I couldn't. I have an empty screw hole on either side. It wasn't worth taking everything apart just to install those two screws. No one has ever noticed, that I'm aware of.
Later years have a very different setup. You have to push the windlace through a channel. The '47-'48 system takes longer, but is probably easier, although poking holes in the rubber and having it all line up is a pain in the keister.
Good luck.
