The two-tone magazines were not blued at the top to preserve the heat treat of the upper body and feed lips. Two-tone magazines were made through the 1930s, when full blue magazines replaced them. The full-blue magazines that followed still show the temperline under the blue where the heat treat was done. Without checking a resource, I think the bluing process was changed from the gas-blue process to a tank blue process, hence the tank bluing process did not destroy the temper of the body and lips like the gas blue apparently did, and the whole magazine could be blued.
Honesty and embarassment cause me to have to correct my above post. As I was writing it, I had a feeling I was missing something, but it did not come to mind. It is frustrating to have a severe brain cramp that causes one to make a serious error.
After checking out my suspicions, I find I had it backwards on the two-tone magazines. The two-tone magazine were first full-blued by the gas-blue process, then the tops were given a three-minute bath in liquid cyanide (!) at about 1475 degrees to temper them. That removed the blue. Just looking at the way the temperline looks, one can imagine the effect as one can see the evidence of tiny "splashes" on the transition line.
The process was dropped in the late 1930s after the magazine steel type was changed to a steel that would not lose its temper in the bluing operation. The full-blue magazines that show the temperline under the blue are leftover former two-tone magazines that were tank-blued after WWII to match the darker blue of tank-blued Post-War guns.
There, I feel better having corrected the record. Sorry for the misinformation. I try very hard to be accurate and do not comment on things about which I am not sure. I was sure, but I was wrong.