Note: I'm asking for a friend.
So a friend of mine had a grandfather who passed away recently. He left my friend a couple of morgans,
Peace dollars, and a few rolls of misc 90%. While helping him look through them 2 coins caught my eye. The first was a 21' Peace (Authentic). The second was an 1878-S morgan. It had been polished and was very apparent, my friend adimitted to doing it a few years back "to make it pretty again

". I flipped it over and wham, as plain as day the lock knock variaty. It's an easy AU Details coin (sorry no pics for a bit). What would you ballpark it's price at? I'm no good with these rare morgan variaties.
Thanks!