Nice find--I agree on the ID as a Cinelli
Well, if you really decide that it is too big, give me a call--reminds me of my long-lost Cinelli "B"--although it was rattle-canned tractor yellow. No unusual holes though. :>)
I was thinking about the BB--a good brazer may be able to tack braze an insert into it--maybe only spanning the gap and inside the threading, and fill braze on the outside. Not the most elegant, but if you worked slowly to keep it from getting too hot, it might work.(My pops who was a great welder and brazer would be your guy except he's no longer with us.)
Cheers.
Dean Kernan
Pomona NY USA