Came across this mini sheet of 20 Scott 2394 with a First Day of Issue of 10/04/1988. This sheet cost the original purchaser 175.00. They were cancelled at a Post Office in Waterford, WI in April 1989. These have never been used as they still have full gum on reverse. I have seen sheets with a First Day of Issue cancel but never a sheet like this. It isn't a First Day of Issue so what could possibly be the reason a stamp collector would have had these cancelled?
Possibly were legitimely used on one parcel going a long way? I Have had a sheet cancelled on item to pay postage rate, and then covered with a plastic film taped over it to prove payment while in transit. When received just cut around plastic and voila, you have a full used sheet of duplicates you can trade or sell and they have done their postal duty.
I think the first solution is the more likely. When a business uses "No Postage Necessary" postcards or envelopes, the local PO will total up the postage charges for all of the returned cards/covers regularly (probably monthly) and then the business gets a bill (postage due). The business pays this bill and the PO actually has to cancel stamps to balance their books. These stamps are usually never attached to anything and given to the business. They are canceled like in the original photo.