what kind of siding is this?

[John Raabe]

Board sheathing and the let-in bracing required to hold the wall up is pretty much a relic of the past.

The installed cost of board sheathing and let in bracing (wood or metal) is much more than OSB sheathing. Considering boards are inferior structurally and will be covered up anyway, why would anyone build that way today.

Answer, they don't.

In some areas they use no structural sheathing except at the corners. In those cases metal strapping is used to give some racking resistance to the wall.

[Amanda_931]

Interesting.

I came across the the 1998 edition (completely revised and updated, it swears on the cover) of Charlie Wing's Visual Handbook and it still lists 1x4 let-in bracing as a option for a lot of situations.

1-story, or the top story of two or three, in all seismic zones.  

In higher seismic areas, structural sheathing for everything else. (e.g., if you were going up three stories only the top story could have let-in bracing).

So, yep, it was in use, and if you are in the low-danger seismic areas, minimal codes enforcement, and did not want to use  sheathing for whatever reason you might still be OK using the let-in bracing with stud construction.

Although some sort of timber framing, real or made up from 2x lumber might be a better idea.