Four score and seven years ago, our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
sbj: fathers
det: our
vrb: brought forth
obj: nation
det: a
adj: new
adj: *conceived*
adv: <in>
obj: liberty
cnj: and
adj: *dedicated*
adv: <to>
obj: proposition
det: the
adj: -that-
sbj: men
det: all
vrb: are created
cmp: equal
adv: <on>
obj: continent
det: this
adv: ...?
I'm working on this but can't wrap my head around the phrase in bold. I know that it's adverbial phrase of time, but what kind of phrase is it exactly? What part of speech is 'ago'? Sources don't seem to agree.