I have done a little more research on the 1941 ID page here, and it appears the receiver was made from 2 parts welded together. Action milled or forged, and the barrel shroud made from heavy gauge sheet steel. My copy of Johnson Rifles and Machine Guns, 1968, ed Don B McClean, page 14, shows a full length receiver, set up on milling machine. I'm not sure if this is just a publicity photo with a finished receiver on the machine, or whether it is a true reflection of the actual manufacturing process?
I thought one of the advantages over the Garand was that it was easier to manufacture the Johnson? This process sounds more complicated than the forge/finish milling of a Garand receiver, which is much smaller due to not having an integral barrel shroud.