Interesting gun.
My take-
The grips are elephant ivory. They are almost certainly for a K frame.
The gun was rebarreled. The O is a service mark sometimes seen on replaced parts.
The ejector rod was replaced when the barrel was.
The gun has been refinished by the Factory.
Did any of you catch the matte finish on the sides of the front sight? That became normal when the Factory went to ramp sights. Half moons were normally polished like the rest of the gun. A VERY few half moons originally had matte sides on some very late assembled 5 Screw guns that were slow moving models.
It is sometimes seen on fixed sight guns refinished late in life.
The frame is not cracked- that is oxidation darkening where the nickel is worn. You can see the same darkening on other corners and sharp edges.
What really puzzles me is the way the butt is marked. I've never seen or heard of a commercial 1917 with a number below the 160,000s.
Eagle heads on the frame and cylinder do NOT help us know if it was a military or commercial because leftover parts used on commercial guns often have them.
My best guess is that was a military 1917 sent back to S&W for a new barrel and a nickel finish. The logo was added at that time. For reasons we will never know, the original butt markings were removed and the original serial number re-stamped. Maybe the guy was a Marine and did not want his gun to say U.S. Army.
I'll be very interested in the frame markings under the grips on the left (thumb latch) side. I suspect we will see a 1970s date, but maybe 60s.
That gun is definitely worth a letter just to see what it began life as!!!