Ideally they are a plug in fit with a step that meets the end of the carbon tube.
I have seen ones on Ceeway that look exactly like this but I'm not sure what they were actually intended for. They might be steel.
Nick Crumpton has done a buildalong showing plug in type bonded dropouts. He likely had his specially made.
In practice a longer tang of metal can be suck inside a tube with a blob of strong epoxy and hold up forever if the end is tapered and wrapped with tow. I think some of Calfee's tow wrapped carbon frames look more like this. Really just like putting dropouts in bamboo tubes. If they hold up in bamboo they will hold up in carbon.
There was a guy on Velocipde putting out feelers to see if anyone was interested in properly made ones. Not sure it anything came of that
