Congrats, I too believe everything seems to point to the 'real deal' so far including:
In particular I see an O following the barrel serial #.
I agree with Masterpiece that there is also a partially stamped sideways N (because it's a curved surface) stamped in the barrel shroud after the serial #.
Most of the Pintos were blued guns already produced, then taken from inventory to be reworked into Pinto models in the service dept. At the time your gun was produced/reworked the Service Dept was in a separate building known as the Outside Svc Dept. Work preformed there before the gun was sold did not get a date stamp on the left side of the grip frame, as one would if returned by the owner to the factory for rework.
When the outside svc dept needed work done in the factory like plating a blue barrel with nickel, they stamped the barrel with an O for Outside to assure that it came back to them instead of being put back into assembly like the other 200 some odd barrels blued or plated that day. When the Service Dept. was moved back indoors, parts sent from there to the bluing/plating departments were stamped with an S for Svc Dept. instead of the O.
Your excellent photos really help! I think you got a winner at a winning price!