I have an old Braun Hobby EF300, circa 67 that has a recessed 2 pin connector, similar type of connector that goes to a bipost shutter sync terminal , but it is indeed a AC line socket.
Adjacent to it is a turn switch to select 120 or 240. It will of a sort work like a battery eliminator, in that the oscillator will run without batteries, but the recycle time is something likel 6 minutes in this configuration. It is for strickly charging the batteries. I have fed it 4x 2V gelled lead acid cells, since the wet cells it was made for stopped production in the early 70's. I think I talked about the restoration in a long post here a couple of years ago.