I had to replace quite a few missing bolts during my engine rebuild. What I found was that Mitsu had lots of their bolts custom made, and the majority have smaller heads than standard. Of course, you can do the majority of the work on my truck with 4 wrenches and sockets: 10mm, 12mm, 15mm, and 17mm. However, when I went to 'fill the missing holes', I found that most of the standard sizes had head sizes that were 2-3 mm larger than the standard Mitsu size (ie a 10x1.25 that had a 15mm head diameter was replaced with 10x1.25 that had a 17mm diameter head).
I think part of it was engineering design (makes them awfully easy to work on), and part of it was cost savings (save a pound of hardened steel per truck times 100,000, that's quite a bit of money).
But, a jump from 17mm to 24mm? No, they were just guessing and got it wrong. <img src="/forums/images/graemlins/rolleyes.gif" alt="" />