The pressure plate relies on gas pressure in the rear chamber to work, so this would have had the opposite effect AND show signs of burning. Another option may be - wait for it - muzzle-loading. If the cartridges were not available, or the breech block seized the carbine COULD still be loaded in the old way - from the muzzle using a ramrod, but that would still show burning.
More likely, if it was seized, or the person had no idea of how to work one of these 'old rifles', any attempt at cleaning the barrel would go no further than the gas seal - and would leave any cleaning material residue there. No, not recently, but at some time - using what cleaning material was available.
Another possibility is a barrel blocked with an oiled cloth plug (not unusual for stored weapons). Any inspection would show this and someone has tried to get it out by using a cleaning rod from the muzzle end - but forgot to open the breech first. They would have succeeded after the breech was opened - with difficulty - and the rifle not stripped further. It may be that the plug just disintegrated over time and fell down the barrel of a butt-down racked weapon!