What's supposed to be in that empty slot is a retractable male buckle. Very strange that it's missing. Anyway, a quick check on Rockauto and e-Bay failed to provide a replacement. And, it appears it's been discontinued from Ford. This is one of those cases where a trip to a junk yard might be necessary (unless someone on here is parting out a vehicle). But, if you live outside the U.S., I understand that may not be an option.