Sometimes these are hors de commerce, as when retained by the publisher for private distribution, or provided as partial payment/compensation to the printer or bindery. But that’s probably seen more in smaller editions or in things like engravings, lithographs etc.

You also sometimes see the number limitation instead filled with initials, as presentation copies or again when hors de commerce. I have a set of a limited edition where the numberline is filled with the publishers initials. Shortly after I saw a set with the illustrator’s initials. That sorta thing.

Typically these copies are more valuable if they have some designation which denotes that they are purposefully withheld from sale as part of the limited edition.

Left blank though, like yours, they don’t carry the same implication, and are just un-numbered, out of series etc.

They may technically be seen as less valuable than limited ones, but it isn’t as significant a “flaw” as one might think.

An ULPT: unscrupulous sorts could infill with a low number. But they better have a period hand, and the correct ink color. Very shady thing to do, but it is done by some. Unethical, frankly. But there are opportunists everywhere.