I was looking at TM's page for the M4 D Type. They say they used the top line paint to make it appear open, so that people will close it. That's great as far as I'm concerned. I looked at the regular and the D side by side and it is very subtle. The paint edge at the toe is higher up on the D. Hell I should remove a little paint on the regular one and see what happens, LOL!
I used to think about putting a pinstripe on the front edge of a driver crown to do the same thing.