Washington Shermack And Interesting Postage Due Pair.

Hi again. Still sorting through GF's collection and thought I'd share a few scans. The postage due stamps are stamped by Narbeth Pa., but the gum on the back says they are unused. (sorry if the scan doesn't show this) Was it common to pre stamp them? Thanks.
Found the shermack with a bunch of stamps that I'm guessing he was going to catalog somewhere but had a heart attack and died before he could.

Yes, it is common for postage due to have gum. Postage due stamps are considered used if cancelled and often were used by the PO for accounting proposes. You can learn more about the Schermack here https://stampsmarter.org/1847usa/PrivatePerfs.html
Don