the fixing post 21 by fitting the washer 25 in the slot 2110 the spring 23 can be elastically compressed between the cap 215 and the step 123 referring to fig5 and fig6 in assembly the two springs 23 are firstly each arranged in a stepped hole 121 of the two ears 120 and the bottom end of the spring 23 is disposed on the step 123 of the base 12 secondly the fixing post 21 are extended through the spring 23 and the stepped hole 121 from the top surface of the base 12 until the slot 2110 of the fixing post 21 is completely exposed out of a bottom surface not labeled of the base 12 meanwhile the cap 215 of the fixing post 21 compresses the spring 23 abutting against the step 123 with a compression force exerted on the step 123 that gradually strengthens thirdly the washer 25 is placed around the extension portion 212 the hole 253 corresponding to the locking tongue 213 and then pushed upwards along an axis of the fixing post 21 until the circular base 251 reaches a position corresponding to the slot 2110 of the fixing post 21 the washer 25 is then rotated 900 relative to the fixing post 21 or the fixing post 21 is rotated 900 relative to the washer 25 causing upper surfaces of the locking tongue 213 corresponding to the arc sides of the locking tongue 213 to be moved to press against a bottom surface of.