The Campy bolt for normal lugs with an 8 mm hole has a 6 mm thread. All the clones too, Zeus Simplex Sugino etc. Even the Richard Sachs, better metal no doubt but still just M6. I have a Jim Merz Campy clone in 6/4 Ti, but still just M6 thread.
The older style of bolt was 8 mm with an external nut on it, a bit heavier and the wrench to fit it is a lot bigger/heavier, not something you'd want to bring with you. But if you want one, you won't find one nicer than this Cinelli, for sale by
Boulder Bikes
You definitely won't break it!
Another option might be the best of both worlds, the Campy 10 mm seatpost pinchbolt, which has an 8 mm thread. Godlike reliability, light weight, and takes a 5 mm allen wrench.
Downside is you need to drill or ream the hole in the seat lug to 10 mm, and then forever after you'll be stuck with this 10 mm bolt. Hardly anyone else made them in that size, though some triple chainring bolts work in that location. In fact the Campy 10 mm seatpost bolt is hardly any different from their triple chainring bolts, other than the female side has knurling on the seatpost bolt, versus two tiny notches for a chainring nut spanner.
Some frames can't be embiggened to 10 mm, and even if you can, finding one of these rare Campy bolts isn't easy. They were never popular. Triple chainring bolts are a thousand times easier to find, and they work great. Functionally just as good as the rare Campy seatpost bolt, but not as pretty on the backside, not bright chrome plated, no iconic 'Patent Campagnolo' stamping. I put one on my '71 Super Course in about '72, just about the first upgrade I did on the bike. Found the chainring bolt in the bikeshop's junk bin, a lonely triple bolt, good for nothing else when you don't have 5 of them, so it was a free upgrade. (Or maybe I just palmed it without telling the boss. Don't rat me out OK?) The Nervex Pro seat lug was very easy to embiggen to 10 mm. I think I used a tapered hand reamer. That part is still on that bike to this day. I still have the bike, one owner for 50 years.
Mark B