It's a score for a game. Not. Just kidding.
I love word studies.
I don't know the etymology of the word in question. Here's some information.
Threescore means sixty (Three times Twenty [3 x 20]).
Fourscore means eighty (Four times Twenty [4 x 20]).
Sixscore means one hundred twenty (Six times Twenty [6 x 20]).
I will look up the history of the term and comment later.
Hopes this helps.