Fwiw, there are a lot of historical overtones in this episode:
(1) The Philistines, one of the "People of the Seas" who settled in Canaan after a failed attempt at invading Egypt (12th century BC), were known as uncircumcised (Judges 14:3; 15:18; 1 Samuel 17:26) -- whereas circumcision was practiced in Egypt (although certainly not always and not on every male) and in other countries (cf. Jeremiah 9:24f).
(2) Circumcision wherever practiced was originally a passage ritual, associated with puberty and a prerequisite to marriage (still echoed in the case of Ishmael, circumcised at 13, or the stories of Genesis 34 and Exodus 4:24ff). The specific Israelite practice of circumcision on the 8th day is a later (postexilic?) religious development.
(3) The practice of human mutilation and trophies in war is widely attested in the Ancient Near East. When one reads 1 Samuel 18 in this perspective one naturally thinks of "foreskins" as pars pro toto, i.e. actually meaning the complete genitals of the victims.
This is not to say that the narrative is historical. But it is certainly an old story, the details and meaning of which changed with the passing of time.