Of course, the real problem here with Enhancement Symbols is that they are built on an IO-only model... and then lots and lots of exceptions have been created over the years.
There actually is no appropriate border for a Universal Damage set at this point since the only current example is attuned, but as Codewalker points out, it wouldn't have to be.
I would actually propose using the generic TO Damage enhancer... since it too can go in any Damage power.
Further, the current Invention Symbols being used for Set Categories is a bit of a misnomer already... because the store sells attuned versions of many enhancers that simply AREN'T invented.
Regardless of how you look at it, there are several now well documented bugs with the latest Mids version... I.E. there are several threads that talk about them now. Note to the OP, please look for other appropriate threads BEFORE you post a new one with the same content.