Well, we know that the majority of the '73's were in 44wcf, and we know there were some checkered pistol grips, and we know that you could order pretty much any barrel length you wanted, so I expect the 20" octagonal with pistol grip probably exists, even if not many. I've even seen a pistol grip '73 with octagonal barrel and double set triggers that was shipped to a Texas Ranger, although I don't recall the barrel length.
I know some folks have complained that the lever on the Uberti pistol grip rifle is more curved than originals, but it is a very minor difference.