You can mouse over the area to see the stiffening plate and hinge. I've also see sections of twinlock or more usually T slot shelving used with a stiffening plate on the back. More usually T slot as it wont take vast weight. [You must be registered and logged in to see this image.]
The downside to using T slot is that the shelves have to be wedged to make then level from front to back. I think the hinge shown has been crafted to not quite fully open.
Obviously their own man the builder of that shelf made the only above window shelf I've seen with below shelf supports.
Having his own (or one's own) independent thought is a great thing but there is little point in reinventing the wheel.
These threads are good for shelves one shows the plans by the late S McVicar for his version of the over window shelf and the other debates a shelf that we seem to have lost the picture of. However the second page shows other possible shelf designs. I think both the chain supported (or cord) shelf and the twin slot device could both be strung from U brackets a la Mr McVicar. [You must be registered and logged in to see this link.][You must be registered and logged in to see this link.]