As requested

 is a note that is found in the Hollowed-Out Rock south of Megaton, together with a Sniper Rifle and other useful items.


 * The "S" mentioned might be the character Sam Warrick, who roams the Capital Wasteland with a Sniper Rifle, usually hunting Wastelanders.


 * The sender who is named "E" may be Eulogy Jones from Paradise Falls.


 * The phrase 'If anyone asks where you got it -- say it was a gift from your grandma' could be a cryptic reference to the G.O.A.T test in Fallout 3 in which Question 5 asks how you would react if your grandmother ordered you to kill 'another vault resident'.