Ink Drop

I have a two part question about a first printing Special Printing

I have an O57s, a 1 cent State Specimen with what appears to be a drop or splatter of ink on the left side of the value numeral. Close magnifacation shows some additional specks of ink on the E of ONE. The print doesn't look like soupy ink ( too much solvent in the mix ) so how would there be dripping ink near a freshly printed sheet?

There are also a couple of owner's marks on the back, one a small stamp and the other a painted on couple of I think that they are letters. When was it common practice to stamp a stamp with owner's marks?

there are some collectors who still stamp their initials on stamps.

Yes, it was common for owner's (and experts) to hand stamp their ID on the backs of stamps. The only danger, of course, is if that ink runs through the paper and becomes visible on the front of the stamp. Then it is considered a fault or defect. Otherwise it's harmless.

George Howard used the same color ink as the stamp to minimize effect bleed through, if it did happen. Green ink on 1 cent stamps and red ink on 2 cent stamps. Pretty clever !