The same catalog states that the frames were heat treated "to a tensile strength over 90 tons per square inch. This is far better than the usual case hardened frame." 90 long tones = 201,600 psi (possibly a bit of marketing hyperbole!)
There is no mention of heat treating the barrels
Edwin Pugsley in a letter to F.W. Olin April 11, 1932
“Both (Model 21) frames were heat treated, then one was case hardened and the other blued. The case hardened frame had a tensile strength of 94,200 psi, an elastic limit of 85,400 psi, and elongation of 2%. The blued frame had a tensile strength of 174,600 psi, an elastic limit of 160,950 psi, (and) an elongation of 12%.”
By the late 1920s AISI 1040 was fairly standard for U.S. maker's double's barrels; including Crescent. Frames were 1020.
Resulfurized AISI 1137 (“gun barrel steel”) & 1144 low alloy carbon steels are commonly used for modern shotgun barrels and are easily machined. 1144 has an industrial standard tensile strength of 108,000. Neither would have the corrosion resistance of 4140.
Up front, I will state I know little of the science of steel, steel types, alloys, ect.
But I do find most of this thread interesting anyway.
As an engraver that has cut many Mod 21 and Mod 70's I can add this if it means anything.
The Winchester (original) 21 frames are (heat treated) so that they were as tough as most anything you would care to cut with a hammer and chisel.
They file, drill, and mill cut with little noticable difference from other steels that you run up against.
But attack it with a chisel,,and it will easily break the points with regularity over and over again.
Resharpening sometimes every minute is not uncommon.
Use of carbide gravers helps immensely, but even then they get battered and shatter the point as well. A carbide point is a bit different in the way it breaks as it can still leave you with a raggedy sharp surface. So sometimes the engraver just keeps going, but it shows.
Use of an air-assist engraving tool further prolongs the point life in the cutting of the frame alloy. The ability to adj to a much lighter 'strike' but yet rapid rate can make the work at least progress at a seemingly acceptable pace.
The toughness of some of the frames was so tough that when trying to do inlay work, the slender inlay punches would just turn over when struck. When reshaped to a stronger angle, the frame alloy wouldn't play along. Instead of under cutting by the punch, the alloy would chip off those thin edges.
The trigger plate on the M21 I'm told (I don't know for a fact) is of the same alloy as the frame.
That trigger plate is as soft to cut as CRS.
Absolutely no problem in working on that part.
But the frame, toplever, forend iron,,all fight you all the way to the end.
...and they are not consistent in their toughness. Some are tougher than others and some quite noticably easier to cut.
M21 bbls are tougher than most other mfgrs bbl material. Not anywhere near what the frame is, but not as soft as the trigger plate of the 21. Nor as soft as say a set of Fox or Parker Bbls.
The M70,,the recv'r is as hard or nearly so as most M21's in the front recv'r ring back to about 1/4 to 1/2" beyond the ring itself. Then it gets noticably easier to cut.
Even the WP proof mark stamped in that area is usually weak on that harder surface than the one on the bbl right next to it.
...and FWIW, the M42 frame is a little tougher than the M12 as far as how it feels when you engrave it with hammer and chisel.
The Magnum rec'vr of the M61 is also that way when compared to the M61 s,l,lr. Not a big difference in each,,but noticable.
I'm guessing the same alloys used and just different HT,,but as I started out,,I'd just be guessing.