Yes, this is a hooked breech and the tang portion should separate easily by levering the tang
downward upward. Then the threaded portion can be removed like any breechplug. Use a very strong vise, and clamp the barrel an inch forward of the breechplug so as to not be clamping the threads. Protect the barrel with sheet brass, sheet plastic, or whatever is incompressible and not thick. Get a long handled crescent wrench of monkey wrench on the breech
hook section. Have at it. I often protect the hook as well with thin sheet copper or brass.
If there’s no movement you can smack the wrench with a rawhide hammer, wooden mallet, copper hammer, or the like. Lefty loosey.
If there’s no movement consider an overnight soak in penetrating oil. 1:1 acetone and transmission fluid is the best but use what you have, and give it a go the next day.
If that doesn’t work get rid of all the oil and heat the barrel, not the plug, till water sizzles. From there on it’s up to you how hot you get the breech end of the barrel.
Let’s say it’s impossible but you’re a clever person who can thread and turn steel. Make a “jag” that just fits the coned portion of the parent breech from stock wider than that orifice. Flare it on an arc.
Now file half of it off so it becomes a D bit. Use that to remove the burr.