Update. Finally had to take the plunge and put my original, still in the cosmolene and wax paper wrapper original C-3940 Breech Bore Gauge to work doing what it was built for! After cleaning the new (67 year old!) Breech Bore Gauge of old wax paper and hardened grease in my gunsmith's solvent tank, I checked the breechbore reading - which was roughly 2.5-2.75, if you were to try to be precise. If this is indeed a new (or nearly so) 10 of 1942 SA barrel (reads a "1" at the muzzle") it confirms that by October of 1942 the throat/leade of M1903 barrels was being cut deeper than the pre-war barrels. I had heard that this was believed to be necessary due to variations in ammo types and quality. By the way, my Canadian Ordnance supplied gauge measured perfectly to the gauge blueprint!
So the throat at the rifling is certainly not the problem with the bolt lift.
Last night I did try a US Arsenal "U.S. Rifle, cal. 30 M1903 A1 Complete Profile of Chamber Gauge" from my US Armorer Tool Collection in the barrel, and it did not seat all the way down flush with the barrel entrance. So something about the chamber neck, then? Next to clean/polish the neck area of the chamber. My gunsmith didn't like the idea of using scotchbright (TM), as he advised it "scratches". He sent me home with 0000 steel wool, and directions to wrap it around a .30 caliber brush and have at it, liberally using oil/lubrication. So we will advance cautiously, and use the Scotchbright as Option #2. We will report back as soon as we have done so. CC