I reckon you're spot on with the '77 shout; first time I heard it too
Spot on about gear being used for Red Leb or Morrocan Black as well
So then... 'Spot on'... Lost Scouse Lingo?
I think I've mentioned on here before that many of these phrases seemed to originate in different areas of Liverpool and then either became de facto vernacular throughout Liverpool or got mutated to mean something else, a lost in translation sort of thing.
I really can't remember exactly when
Spot On became common, certainly very early 70's as far as I was concerned, and I could be wrong but it seemed to gradually replace the expression
dead right which I think we used to commonly use in the late 60's.