I need to release a drawer from an old desk (about 1980), but when the drawer is fully extended on its metal slides, there is no release tab on either slide.

Hi endese and welcome, does this drawer extend fully? Should be a release on the side. Can you post a picture?

Yes, side release particularly with this vintage. Extend the drawer and look for a metal tab, sometimes black, sometimes metal colored at the rear of the slide. Push the tab out, release the slide and the drawer should come out. Others used an off centered catch, pretty visible, rotate to center position and pull drawer.