It's unlikely it was actually a "bring back".
The adjustable rear sight wasn't common on Carbines in Europe, being a very late war item that just didn't make it to the ETO much before the war ended.
Likely, this was added to the Carbine as part of a post-war rebuild program.
During the 1960's, a LOT of M1 Carbines were sold by the Director of Marksmanship program through the NRA.
These sold for $20.00: $17.50 for the carbine, $2.50 for UPS shipping.
In the mid-60's these flooded the surplus market and were very common.
A lot of people get confused about "bring-backs". The only way to bring back a USGI weapon was by stealing it.
People would sometimes look the other way on 1911 pistols, but they were Hell on people trying to take off with M1 rifles and Carbines.
Where the confusion comes in, is a lot of vets bought surplus GI Carbines and later told their kids "This is the rifle I carried in the war". What they really meant is "This is the TYPE of rifle...".
The kids took it to be the very SAME rifle he carried.
I once saw a near fist fight at a gun show over whether a National Ordnance 1960's commercial carbine was the actual carbine a man's dad had carried during the Battle of the Bulge.
Again, the old man had SAID "This is the rifle I carried....".