Xserve racking kit missing the front small block

Hello fellow mac users

Recently I acquired few Apple Xserve with all the racking rail kits. Only when I tried to install the rails I noticed that the small piece of metal that holds the rail kit in place are missing. The small piece in question, which the Apple documentation call it "spacer". I have a Sun rack, which is a threaded-hole rack. Any ideas of how I can install those rails w/o the "spacer"?