Missing Period On Fargo N. Dak Bureau?

I have been going through an old album (so old it doesn't lie flat and I can't work out how to open it to remove pages - hence no scan).

Under Fargo, ND, there is a strip of 4 x 1c Liberty each with a Type 61 precancel on it. The precancel is very well printed, not at all faint. Three of the stamps are normal, but the leftmost stamp is missing the period after Dak

N. Dak

I recognize that it could just be an anomaly of the printing and mean nothing. But the typed description with the stamps says that this is an example of an error (yet there is no mention of such an error in the PSS Catalog).

I don't really collect errors (have to draw the line somewhere), so I'd like to work out what is going on and then dispose of the strip.

Does anyone know anything about this?