O2 Story
 

In case you hadn't!seen it...
http://airfactsjournal.com/2016/06/p...it+Of+A+Cessna

Good story... these FAC's were amazing.

Very interesting read for sure...

It also makes me wonder if 337's wings are same internally as O-2's or they have no hard points built into them apart from hinge point for wing strut?

I'm sure someone will correct me if I'm wrong, but I believe that internally the hardpoints are there. You just need to have the mounting hardware attached. Aviation Enterprises in Nashville, TN is one company I know of that can do this. They use them for mounting external auxiliary fuel tanks on the Skymaster.

Probably thread drift but has anyone installed the external tanks from Aviation Enterprises or done business with them recently?

The wings are structurally different. The attach bolts are two bolts sizes larger, there is a steel strap on the spar to absorb the firing loads from the external hard points, the pitot tube is mounted much further outboard on the left wing to make room for the hard points, i.e., different part numbers which would make a major alteration