According to these guys, the 6 groove barrel was made by High Standard from roughly Jan. 1943 to April 1943. Supposedly only 5,000 of 'em made too. The S/N indicates your rifle was made in 1943. I think that's an original barrel.
There were no SC '03A4's. SC did make NRA rifles, but not with that kind of stock(there are all kinds of pictures of full military stocked SC's with the Redfield sights). Cousin Bubba, methinks.
https://www.rockislandauction.com/de...groove-rifling
You have any 168 grain Match ammo? I'm wondering how well it shoots with better ammo.