My understanding as a machinist, not a metallurgist, is that heat treating doesn't begin to give much increase in hardness & strength until about 30 points of carbon are reached. The 1020 frames would have been carburized & given a hard case with little effect on the core of the frame. This was in fact I believe the intent.
Realize it doesn't take much carbon to have effect. The 40 points of carbon in a 1040 steel amount to 0.4% by weight. Thus for every 100 lbs of iron you would have only .4 lbs of carbon or 4lbs per 1,000 of iron.
