Actually, no. I was not referring to socket attachment, which... yes, is not for attaching Static or Interp. No, I was referring to the one in Actor Properties, which requires you to select the actor, lock the properties, then search for a variable called 'Base' in Attachment category. It is an actor variable, then select the actor you want it to parent on and insert it into the variable.
https://udn.epicgames.com/Three/AttachingActors.html
Refer to the 'Base Pointer' section