This "setter's aid" tool is a set of 2 adjustable collars which
fit a standard bur shank. With an allen wrench, which is included,
you can adjust the distance between the cutting edge of the bur and
the end of the "collar". This means that by adjusting the setter's
aid on a suitable setting bur, you can use the bur to cut a groove
around the inside top edge of a heavy walled bezel.
Oh! THAT’S what it is! Well, it’s a seal at $12, but you can also
make a similar tool for the price of a pencil. Cut about an inch long
section of a pencil, file the ends nice and flat/square to the shank,
and drill out the “lead” with a bit the same size as your bur shank.
Slip it onto the bur, and insert it in the flexshaft so just the
right amount is sticking out, and there you are, ready to roll!