Black skin under cockpit windows.
 

In the 70's, it seems most jets had a black painted area directly below the cockpit windows. Nowdays none seem to have it.

Can anyone explain why this was done in the 70's.

I suppose this was done to prevent glare - the black paint would not reflect the sunlight into the cockpit... Nowadays usually the area below the windshield has a matte finish to do the same thing.

.. and probably also due to improvements in windshield technology to minimize glare these days...