I had this exact problem. I ordered a "Campagnolo" drop bolt from an eBay seller in Europe and it turned out to be a Modolo bolt that had too large a diameter. I had difficulty sourcing a proper bolt and eventually bought one from an eBay seller in the US (southern California I think) who makes them out of titanium. That one worked perfectly, though it was a bit pricey. I bet I saved a whole 1/2 oz of weight too!