Ian, the 6,3/135 Tenastigmat in the auction I linked to above ( https://www.ebay.de/itm/203059183602 ) has an outside diameter of 30 mm, just like your Goerz-Ihagee Doppel-Anastigmat. I asked the seller and he sent me a picture of him measuring it with a caliper.
Dogmars were sold both in barrels and in shutters as well as on cameras, but I think the Tenastigmat was for the Tenax cameras only, same as the Tenaxiars, and that's why they were so named. Dogmar was a well known name and a respected lens, so I think both Goerz and Ihagee would have been reluctant to rebadging it. But with the Tenastigmat it's a different story. It was named for a series of cameras and had less "prestige", so it could have been rebadged for Ihagee. Maybe the Tenastigmat was an updated version of the Syntor (6,3 instead of 6,8)?
Dogmars were sold both in barrels and in shutters as well as on cameras, but I think the Tenastigmat was for the Tenax cameras only, same as the Tenaxiars, and that's why they were so named. Dogmar was a well known name and a respected lens, so I think both Goerz and Ihagee would have been reluctant to rebadging it. But with the Tenastigmat it's a different story. It was named for a series of cameras and had less "prestige", so it could have been rebadged for Ihagee. Maybe the Tenastigmat was an updated version of the Syntor (6,3 instead of 6,8)?