the outside diameter of the bolt is about 5.9mm, the inside about 5, the calipers I have measuring to .05mm. it fits the bike eyelets fine (bianchi strada). Both the rack and the bike have 5mm inside diameters, but the rack doesn't appear to be threaded...there are faint marks that are treaded form putting something in, but not deep and not treaded all the way around.

My guess is that the previous owner just used a smaller bolt that had a 5mm outside diameter or just above it, and perhaps used a nut on the other side.

I figure If I just drill a 5.9 or 6mm hole, it shouldn't be a problem because it will be sandwiched between the head and the bike eyelets.