Well I'm not sure, but I don't think my original guess was right. I was thinking of a metal piece with rubber inserted that goes under the door garnish molding at the bottom of the window, similar to the model A's (as in picture below). It's hard to visualize the shape of the one you have - in some pictures it looks like there's rectangular cutouts where it gets attached, and in another one it looks like round holes only(?). Can you get some good closeups of an end view to show what the shape of the molding is?
As I said, I'm not familiar with your model, but everything else with the Model T moldings is so similar to those on Model A's that I made a guess. Also doesn't seem to be the right number of holes - does your door/garnish molding have 4 screw holes as in the picture, or only 3 as there appears to be in the mystery part?
Can't think were else a piece like this with rubber inserted would go. Are you sure it's Model T and for your body?
Seems to be something wrong right now and my second picture won't upload.
Anyways, good luck. Hopefully someone can help you out.