Wing spars: Model 1 vs Model 2

[foxfever]

I am wondering if anyone here knows what the difference in the spars are from a model 1 to a 2?

Was considering using model 1 wings on a model 2, bad idea?

The history shows that with 2 came a stronger spar, but what makes it stronger?
Strut attach bracket? stiffners? wall thickness? Diameter?

thanks....

[FoxDB]

The Model II Spar was extruded with a center rib the full length and a varying wall thickness .083 top and bottom and .065 front & back. The model I has .065 wall with no center rib.