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Recessed Brake Mounting
Caliper brakes have a center bolt 6 mm or 1/4" in diameter, which fits through matching holes in the fork crown and the brake bridge of the frame. Traditionally, these bolts have been long enough to protrude all the way through, to be secured by a normal hex nut and associated washers.
Most newer good-quality bicycles that use caliper brakes use recessed mounting to save weight and for a more elegant appearance. Calipers intended for recessed mounting have much shorter center bolts, which do not extend all the way through the crown/bridge. Frames/forks designed for recessed mounting have stepped holes...6 mm in front, substantially larger at the back. Instead of a conventional hex nut that takes a 10 mm wrench, a cylindrical nut is used, which is broached for a 5 mm allen wrench.
See the article about caliper brakes for more detail, including advice on mounting recessed calipers on older frames
