5,1 Dual and Single processor trays

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I have a 5,1 Mac Pro Twelve Core 2.4 (2012/Westmere) and an extra 5,1 single processor tray with a single 3.33 GHz Six Core Xeon W3680. I wanted to swap in the single processor tray while I procure two matching Xeon X5690s to upgrade the dual tray. But the single processor tray doesn't go all the way in. They are both 5,1s, not a 4,1. Is there a trick to getting the single tray to seat correctly? What might I be missing?

 

Thanks!

 

Source: MacRumors