Samson Classics Brass "Harry's Nuts"
New style guide nut, round to eliminate nut driver fit and lead wire interferance. Uses custom driver
They are larger in diameter than a regular 3/8 nut, so the support of the guide is better.
Since the driver drives from the top instead of around the side, they fit cut-down guides much better.
These are CNC machined, so the are flat and accurate, not a production nut that someone has modified to use on a slot car.
They are available in brass and aluminum. The brass ones weigh just over 1 gram each, the aluminum ones just under 1/2 gram each.