Regarding "Heat Treatment" while slightly off topic, but I think relevent Julian Hatcher discuss heat treatment of the "Low number" 03 Springfield receivers in his "Notebook". Seems even the prestigious Springfield Armory at the time was still relying on "Color" observed by eye for the proper temperature. Litle problem was encountered as long as "Experienced Personal" were doing the work, but increased production for wwarfare resulted in less experienced people having to be utilized. Problems began to surface.
No doubt many of the double makers of this era also relied on temperature Color for proper treatment with a resultant occasional error. I do not think these few cracked frames should necessarily be considered "Typical" for a Fox gun. It is also noted I say this, "No"t as a prejudised FF (Fox Fan) as Lefever is my favorite of the American Classics.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra