We don't know what it looks like, and it is important to be sure it isn't a crack. Having it fixed to a tyre at the time is feeble compensation for blowing up an original Trapdoor. I've got a 45 degree angled dental mirror and an inspection lamp I made with a grain-of-wheat bulb soldered to coaxial wire and sealed off with heat shrink tubing. With a little aluminium foil lampshade to keep the bulb out of the picture, I think a close-focussing digital camera would give you a picture you could enlarge for a good look.
Assuming it is only shallow, it might be pitting, or the work of some member of the intellectual classes digging out a stuck case. This might not impede extraction with modern cases and light loads, and with an original Springfield they ought to be light. To remove a case that does get stuck on this trial, use a metal rod from the muzzle.
If it does cause an extraction problem, see if it is any better when the hole is filled with hard, cold candle-wax, for that is incompressible, and won't be squeezed anywhere at the speed case expansion happens. It should prevent expansion for a single shot.
If the wax is any help, clean the pitting very thoroughly with soapy water or alcohol and a wire brush. If there is rust in the hole, naval jelly rust remover will remove it easily after degreasing, with no damage to the chamber, but don't get it on the blued surfaces, and don't use any product that claims to leave a protective layer. Then fill the hole with either soft solder or plastic steel. Don't overheat the receiver, but the melting-point of soft solder, or the temperature required to remove unsuccessful plastic steel work, won't do that. Both can be scraped flush with the chamber wall, using a piece of sharpened steel rod or tubing.