http://www.classicfasteners.com.au/T...a/IDENTGID.PDF
http://www.classicfasteners.com.au/techdata.htm
Measure the diameter of the bolt in question. If you believe it is imperial, set your vernier to measure in inches.
Then measure the TPI to identify the thread used. Then order the new ones from Namrick.
Of course, if you replace the bolt and the nut, it's much more simple. Just get the correctly sized imperial bolt, with matching nut.
If it's about 12mm, but 12mm is too small, then I'd guess it's a 1/2" diameter bolt. But measure just to be sure!