That is a die crack on the reverse of a Pilgrim half dollar ... you mentioned 1920 in your post title ... and as such there were 152,000 of these distributed (with 200,000 minted).
I mention the mintage figures because a die crack occurs ... obviously ... near the end of the useful life of a die ...
Not sure of the 'useful life' of a commemorative die in 1920 ... but at least 50,000 coins each ... so your coin was struck near the end of one of the dies used at the time.
Nice example ... show us a photo of the entire coin and we can offer more informed opinions.
David
Take a look at my other hobby ...
http://www.jk-dk.art