stamps.com stamps can be printed by customers on their own printers. stamps.com sends you the labels, you access their website with your account, and then you print as needed. Each time you print, your account is charged for the postage.
I am unsure how this would be classified since it is quite easy print errors at will using their label stock. Don
It appears that the person just put the label on the letter without running it through their printer. It is an oddity, because the Post Office canceled it, instead of sending it back for proper postage.
No doubt the blanks are tagged and that is all it takes to pass properly through the facer machines. A stroke of most hi-liter pens will do the "tagging" trick too - very useful to go along with older stamps.
Being a cut, it is speculation how the cover was handled by the USPS. Was it caught or not? If caught, was postage due ever collected from anyone? Did it have enough valid postage next to it for its journey? I'll vote "no" for all of these. Being a customer-produced error, it has virtually no additional value ... especially as a cut, having lost its postal history story.
It made it to its destination. Unfortunately, this source will only give me the cut corners of the envelopes. Good that I get a nice supply of modern used stamps. Bad in that some should have been kept as full covers.