Question About Gum Application, 1920-S US Stamps

Greetings Everyone:

I'm looking at a 1920s vintage U.S definitive. I don't posses the stamp and am unable to post photos.

Was gum always applied across the entire back of stamp sheets, or was it normal (on occasion) for the outer dimensions of the selvage to NOT receive gum? Stated differently: If encountering a "mint" stamp that is completely gummed but the selvage is only partially so - can I be assured that this is a genuine specimen?

As always, thanks in advance.

That is normal.

Sometimes even some of the stamps at the edges of the sheets did not get fully gummed.

This is typical of stamp sheets that were wet printed on damp paper and dried before being gummed. Wet printed stamps are gummed after printing. U.S. postage stamps were wet printed until the 1950s when dry printing was introduced.