Originally Posted by
daka
One of the framebuilders who post here will be able to give you a better answer, but if it were my bike I would drip a little epoxy in that gap to help prevent water from getting in there and causing corrosion. I suspect that there is enough area brazed together to ensure that the seat lug doesn't detach from the tube.
I would also see if I could find the washer that belongs under the nut on the seat post clamp so that the nut doesn't continue to grind up the lug.
I would not put anything in the gap. I'd bet that when the proper size seatpost is in and tight the gap disappears. A lump of epoxy could well mean you can't get the correct post in - or if you do and tighten it properly either the epoxy crushes or it distorts the outer part.
And it's not likely a structural issue - not with the reinforcement of the seatpost, and besides, there's a bloddy great slot through the whole lug right next to it.