Here is another use for spacefillers. Scott and Yvert list a color variety for this stamp. Also, the '5' and the 'F' were added in a separate printing step, which gives rise to variations in color and size, and a variety where they are omitted altogether.
A person could be a 'serious' collector who wanted to show some variations, but didn't want to invest thousands of dollars in this one catalogue number to do so. And in the case of the 'omitted' variety, both Scott and Yvert mention that all known examples are damaged, anyway.
I would be more impressed with, and interested in, a page built around 5F variations, faults and all, than a preprinted album page for the 1863-70 series with one basically-sound stamp in each box. (And I would suspect that the 5F was thinned, in any event.

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