BahrainLad...
Wing Fences.
These are the scimitar (blade not car) shaped devices fitted to the upper surface and leading edge of wings on aircraft such as the BAC 1-11, DH 125.
Usually situated about half way between the wing root and tip, they reduce the spanwise flow of air from root to tip and increase the efficiency of a swept wing.
Boeing do a similar job with little bits of metal stuck to the top of the wing, these are known as vortex generators and do the same job as a wing fence but in a different way.
The devices you describe with a "an equal distance below the wing as well as above", I think are referred to as 'end plates'.