HP J3245A Installation And Configuration Manual page 27

Installation
3. Mount the Switch
Notice that for the first AdvanceStack device mounted in a rack, the
bottom notch and the top hole in the bracket are used. For the next
AdvanceStack device above, the bottom hole and the top notch are used.
(Refer to figure 1-11, below.) Continue to alternate "notch and hole, hole
and notch" for all AdvanceStack devices to be consecutively installed.
Figure 1-11. Correct Rack Mounting for Multiple AdvanceStack Devices
1-13