Rarity is in the eye of the owner. I communicated with the seller on Trapshooters.com and he sent me photos of the barrels. Indeed, they do look uncut with the blank in front of the bead. He states that Babe Del Grego pronounced the barrels original.
The gun SN is not listed in the serialization book which suggests (and this is only my theory) perhaps the gun was sent to another side of the shop for the 26" barrels whence it was completed, and the info was never given to the scribe of the records. Since the 26" barrels are a "one off" they are rare in the fact that many other guns with SN on both sides of the 26" gun are either 28" or 30".
