I experienced the same thing a few years ago, so I thought I'd post a link:
http://www.fourwheelforum.com/showth...ghlight=thread
There was a switch from the JIS to the ISO standard threads in 1967, which affected the 3, 4, 5 and 12mm sizes. Check out the link above for details.
With my CA95 petcock threads, I simply drilled and re-tapped for 6mm threads and it's been great.