Which DSM version are you using?
I was using 4.3 3827 and it worked then, now using DSM 5.0 4458 update 2 and still working
It also comes with the low profile bracket which you need.
To add a serial I edited the menu_alpha.lst file and added the serial number in the section as below
title gnoboot-alpha
root (hd0,0)
kernel /zImage sn=B3JN12345