It is not uncommon on older Fords with glass fuses to find a space with only one clip. It was unused in that configuration so they did not install the other clip.
It is also easy to miscount or mis identify the fuse positions due to positions of the fuses, staggering of them and the difficult to read info molded into the plastic on the edge of the opening.
Charles
2007 Winnebago View 523H on a 2006 Dodge (Daimler-Chrysler aka Mercedes) Sprinter 3500 chassis (T1N). Bought Sept 2015 with 18K miles on it, Prog Ind HW30C, Prog Dymanics PD4645, Chill Grille, PML/Yourcovers.com deep alum trans pan, AutoMeter 8558 trans temp gauge, Roadmaster sway bar, Fantastic Ultra Breeze hood, added OEM parabolic mirrors and RH aspherical mirror.