Like you, I do not remember reading that "counter boring" was a military thing at all - if the rifling gets worn out at muzzle, I suspect most country's military would have condemned it? Sold off cheapish - then someone buys it and attempts to extend the life by counter boring?? Creates the effect of the "muzzle" way down inside the barrel, but keeps the exterior of the barrel looking original - I believe was done with most military caliber rifles - 30-06, 7.62x54R, 303 British, 6.5x55 and so on. So if most of the reason to do so was to preserve original exterior, then that grind / scour ahead of the bayonet lugs a bit of a mystery? Is several that I correspond with who feel the USA requirement for importer stamp on barrel "ruins" it for collection purposes, but likely of no matter to make up a "shooter" - your pictures as if someone paid for a "shooter" grade and then thought to modify to a "collector" grade - but that counter bore kind of cancels out that idea - but who knows - was lots of combinations to try to do, and sooner or later someone buys it.
Was also "common" for ex-servicemen to buy former surplus milsurp guns and make up something that looked "better" for shooting a deer or a moose - and they worked very well for that - I assume was done from B.C. to Newfoundland - my Dad was in Saskatchewan when he did up the P17 that he used for decades for hunting.
Being 90% Bubba myself, that muzzle "scouring" might also be from a centre rest on a manual lathe that was dirty or not quite set up correctly - in order to support that barrel when the counter bore was reamed? But the area that you show might be 4 or 6 times as wide as my centre rest "wheels", so who knows - something was moving? Maybe nothing to do with a centre rest at all ...
There was a time - 1950's / 1960's - when those barrels likely cheap like deck screws are today - and even small town hardware stores would have stacks or barrels of complete mil-surp rifles - so at the time, was like something to "play with" - to try stuff - often did not work out well - then, today, we think is something "old" and "valuable" - was not thought that, some years ago. I have read of buildings with milsurp rifle barrels in the concrete - cheaper than re-bar at the time - or perhaps more readily available - not likely to be done today, but apparently that was an attitude toward them that some had.
