But I think I have it now...
The wood panel (a thin wood panel glued to frame, the open frame back allows the hanging wire to be easily attached).
It provides a stable firm gluing surface to attach the linen or cotton to on top and when wrapped around all sides.
Here's how it looks while mounting (this will give you an idea of the dimension and look of the drawings that are mounted this way).
The wrapping around the sides extends the image and gives the work a very finished looking presentation.
So what do you think?