Aaron-

The chronology of pre-64 M70 barrels in 375 (H&H) MAGNUM is as follows.  I’m sorry this is rather longwinded, I just pulled it verbatim from something I’d written last year…

“The 375 H&H Magnum chambering accounts for another four configurations of the Standard rifle style. For a brief period, these rifles featured a 24” standard contour barrel and differed from the other Standard rifle offerings only in the presence of a factory-installed rubber recoil pad. Accordingly, the “Wholesale and Retail Price List of Winchester Arms and Accessories” released January 2, 1937, did not differentiate the 375 H&H Magnum Standard rifle from the other Standard Grade offerings except to note that its magazine held three cartridges instead of five.  This variant was very short-lived however, as the excessive recoil generated by the powerful 375 H&H Magnum cartridge fired from a 24 inch standard contour barrel led Winchester to conclude that a heavier barrel was required.

The second 375 H&H variant therefore featured a 24 inch medium heavy straight taper barrel identical in contour to that used on the early Target Models. The change was implemented early in 1937, if not before, such that the 1937 edition of the Winchester World Standard Guns and Ammunition pocket catalog (Form No. 1460; © 1937) noted the weight of the Standard Grade rifle with 24 inch barrel as “approximately 8 lbs., except .375 MAGNUM” and the 1938 edition of the same catalog (Form 1510; © 1938) described the weight of the Standard rifle with 24 inch barrel as “approximately 8 ¼ lbs., except .375 Magnum, approx. 9 lbs.”. 

The third and final 375 H&H Magnum barrel was designed specifically for the cartridge.  The Contour 1 (C-1) barrel was 25 inches in length, intermediate in weight between other two, and omitted the rear sight barrel boss. When so-equipped, the rear sight, initially a Lyman 6W and later the Winchester 70B rollover sight, was dovetailed into a special base attached to the barrel by two machine screws.  The C-1 barrel first appeared in 1939 but overlapped with the 24 inch medium heavy barrel for several years.  Since the C-1 barrel persisted until the end of pre-64 Model 70 production, it was used with both the NRA and Monte Carlo stock styles, resulting in two more configurations.”  

The photo below (from my collection) illustrates the evolution of the Model 70 Standard rifle in 375 H&H Magnum and the resulting four additional configurations.  The 24″ heavy barrel rifle you’re asking about is second from the top.

Same sequence applied to the Super Grades, except that it’s not clear how many/if any Supers were made with the “pencil barrel”…  In Supers, the C-1 barrel first appears somewhat earlier than in Standards.  I think Winchester continued to use the heavy barrel in Standard rifles until they ran out, while putting the “new” C-1 barrels on Supers.  The next photo compares the 24″ Medium Heavy and 25″ C-1 barrels in Super Grade.

As for the exact specs, the MH barrels used the same contour as the 24″ target rifle barrels, and like the early (1st variation) target barrels had a forged integral front sight ramp.  They are straight taper, 1.130″ at the breech and 0.780″ at the muzzle.  They have a flat muzzle (not crowned).  Most of the 375 Magnum barrels have a rear sight dovetail in the same location as a standard rifle, and are dovetailed underneath for a barrel bedding screw (like a standard rifle).  Most of them also have superfluous scope block mounting holes on top of the barrel, same location as the target barrels…

Hope this helps,

Lou