Definitely not a 360No.2 and from the looks of the chamber cast, not really rechambered to anything except that the throat has been reamed out cylindrically, which , of course, Flintfan had mentioned in the original post.
Seeing that now, I would try smoothing out the fore edge of the chamber lead and see what kind of accuracy you end up with. The rifling looks quite good downstream from the chamber.

I wonder if a previous owner experienced some slight ringing in the chambers and had them reamed out to clean them up(?)