Just to be clear, the bag is D-shaped. It’s not a bag exclusively for D-shaped tools. That would be limiting.
An absolute design classic, the D-shaped bag has featured in Brooks catalogues as far back as 1910. Its innovative design features a two-piece construction. The outer sleeve remains permanently attached to the saddle, but an inner purse can be removed so you can carry valuable contents with you. The purse is attached to the sleeve by a ‘Sam Brown’ fixing, preventing it from sliding out even under extreme riding conditions.