The one on the right that has what you refer to as two tabs and an offset serial number is a Tula one.
Even though it may look look like it in the pictures, it does not have two tabs.
That entire rear flange that is spot welded to the gas tube itself is a one piece cast unit where as the one on the left is a two piece, half cast, half stamped part. If you look close at the Izzy one, you can see the part that retains the rear of the handguard is stamped and formed heavy gage sheet metal attached to a cast rear flange spot welded to the stamped heavy gage gas tube.
Serial numbers are the key.
The font used on the Tula tubes are distinctive Tula while most of the Izzy style tubes show the Izzy style font.
In my opinion, Tula did not make a lot of spares. At least not as many as Izzy did because you see a lot of replacement gas tubes and top covers in these kits and most use the Izzy style font.