About March 1944 for the first adjustable rear sights. Then the carbine was sent to a distribution center and then on to a point of issue. Could be in the states or could be in theater that it was issued. Point is this all took time and it would depend on where he was when it was issued to him. They didn't show up in Europe till after Germanysurrendered and the occupation troops fresh from the states had them. There would have been a few used as replacements for sights that had been damaged earlier. Some troops fighting in the Pacific like in the Philippines or Okinawa in 1945 had them.
As Charlie said it was Inland that started using them first. Both the machined type 2 and stamped type 3 were in use at the same time. But they were in short supply. Underwood never did use them. The remaining makers used them if they got them and mostly at the end of their production.