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. This may also refer to the sniper guarding Megaton, Stockholm.


 * The sender who is named "E" is most likely Edward the Cannibal from the Fallout 3 Diaries.


 * 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".


 * This could be a reference to the "hollowed out stump" behind Thirsk, from The Elder Scrolls: Morrowind's expansion, "Bloodmoon". that says nearly the same thing.