From what I've read:
The front of the barrel collar was marked “30-06” in the 12 o’clock position and “41” in the six o’clock position. There were some Johnson barrels made in .270 and 7mm caliber, but these were not used in the U.S. military service.
Read more at Ammoland.com:
http://www.ammoland.com/2010/10/14/johnson-m1941-semi-automatic-rifle/#ixzz25jle1YlJAnd;
hello ,,johnson made barrels and i've seen a few on vintage springfeild bolt action custom rifles built post ww2,,is it a springfield action or mauser action,,or enfield action it's built on? sounds like a rifle built by or for someone with custom features ,,some of these sporters will bring good money depending who built them and the quality of the job,,so as for value it would depend on how nice it is ,,where your located and the market as well as condition..proably in the 400-600+ range depending on the condition and the craftsmanship put into it..possibly more if a well known gunsmith built it or at auction or to a vintage custom rifle collector,,some of the old gunsmiths built some really nice rifles and it sounds like your is a classic example of that!
bigcurt
http://www.gunvaluesboard.com/johnson-automatic-bolt-action-rifle-1819116.html