Warning This Section Does Contain Spoilers
The Friend letter has the line "A beautiful musing on rebirth, I find. l25."
At first look this looks to be one-twenty-five but when looking closer and comparing the "one" to other letters in the document, you can see that this is in fact an "L" and not a one, making this L25 instead. When doing a search for L25 we come to the following:
L25 is the imprint on the drug Pirmella, a birth control pill made from a combination of estradiol and progestin.