You may be taking it too specifically Zimme. Just because something looks like something used on a carrier doesn't mean this was. Even USAF planes have arrestor hooks, similar to carrier planes. Doesn't mean they're used on carrier decks. There were a number of catapult systems used on dry land. There was always an effort to shorten takeoff runs or to use JATOs to overload a plane's weight and get it safely off the ground. Either this was for towing (which He111s were widely known for -- towing gliders) or it was for some kind of ground catapult system. He111s were also known to haul heavy weights, cargo cannisters, and even versions of the V-1 underneath. There are any number of logical reasons to have that kind of fitting on the airframe.
There was no such thing as a carrier He111.