the recessed chamber 18 the two openings 22 are respectively extending through the bearing surface 16 in communication with the recessed chamber 18 the two receptacles 24 are respectively protruded from the bearing surface 16 corresponding to the two openings 22 each having a receptacle hole 24 a respectively disposed in communication with the openings 22 and a pin 24 b inside the receptacle hole 24 a the engagement block 26 has two beveled guide edges 26 a symmetrically disposed at the left and right sides according to the present preferred embodiment the mounting base 10 is fixedly provided at the rear luggage rack 101 of the motorcycle 100 as shown in fig2 the mounting base 10 can be affixed to the rear luggage rack 101 of the motorcycle 100 by any of a variety of conventional mounting techniques the first locating device 28 is formed integral with a part of the bearing surface 16 of the mounting base 10 near the rear end 14 having a hook 28 a suspending above the bearing surface 16 and extending in direction toward the front end 12 of the mounting base 10 the lock 30 is mounted in the front end 12 of the mounting base 10 and extending to the inside of the recessed chamber 18 having a t shaped stop block 32 a qualified key 102 can be inserted into.