visual approach monitor; VAM
As I recall it wasn’t a true HUD by today’s standards. The one which I flew during a demo flight used a gareshield mounted reflector glass to display a visual aiming point (3deg). This was used as a glideslope reference when it was positioned over the intended touchdown point. I don’t remember that it was roll stabilized.
The most memorable aspect was when the demo pilot ‘Flick’ Flicker (apparently a notable character) placed a map in the windscreen to obscure the view and suggested that a landing flare could be made safely. I don’t recall any rad alt display.
It may have been used as a rudimentary windshear detector/guidance system – using the visual references. BA evaluated the system and some of their aircraft may have used it (Trident?)
IMHO it wasn’t very practical, probably oversold.
As for the first HUDs, check Aeropostale Caravelle and the Murcure aircraft.