rolled tabs 58 are arcuately shaped thereby defining a gap 588 between each distal edge of the rolled tab 58 and the side wall 56 each rolled tab 58 defines a first and a second securing holes 582 584 corresponding to each protrusion 18 of the back panel 10 each first securing hole 582 and the corresponding second securing hole 584 are located along a same arc of a curved part of the rolled tab 58 referring also to fig4 in assembling the retention member 50 to the back panel 10 the rolled tabs 58 of the retention member 50 are pivotably engaged with the corresponding supporting members 16 of the back panel 10 through the spaces 162 respectively then the protrusions 18 of the back panel 10 abut against the rolled tabs 58 of the retention member 50 respectively referring also to fig5 and fig6 in securing the expansion card 70 to the back panel 10 the retention member 50 is rotated up until the protrusions 18 of the back panel 10 are received in the corresponding first securing holes 582 of the retention member 50 then the securing clip 72 of the expansion card 70 is positioned on the barrier 12 of the back panel 10 the top portion of the securing clip 72 is pressed against the top flange 124 of the barrier 12 as the operating portion 54 of the retention member 50 is pressed down the protrusions 18 of the back panel 10 are disengaged from the corresponding first securing holes 582 of the retention member 50 the operation portion 54 is continuously pressed down till the fixing portion.