Motor Mount Bolt Size?

I'm resurrecting this thread out of curiosity, not dire need.

I'm installing spool type mounts in a custom installation. The saddles were cut from a "newer" Dodge truck, and the mounts are stock Anchor replacements. A 1/2" bolt fits the hole in the mount, but not the slot in the saddle. The slot measures 15/32", which is smaller than the minimum 1/2" bolt dimension.

My question. Did the factory at some point change the long mounting bolt to perhaps a metric size?

I can simply grind out the slot to accept a 1/2" bolt, so it's nothing more than another annoyance, but I'm curious.