The ventilated rib was introduced on SxS guns around 1925-26 so it is doubtful but not impossible that your barrels are original to the gun. I can't explain with certainty how the barrels got on your gun with all matching numbers. My best guess is that it is a factory rebarrel.
If you look at the underside of the rib with round posts you should see short length dove tail cuts in the rib where the round post joins the rib. If the cuts are present then it would truly be a compensating rib as the cuts allowed movement of the rib fore and aft as the barrel temperatures changed. If the cuts are present then my guess of a factory rebarrel would be reinforced as I doubt an after market set of barrels would go to so much bother.