I think this refers to the prepersonal, anonymous perception that MP refers to as "the one" "l'On" as Kruks explains. This is the prefiguration. See: Kruks 35-37 Anonymous embodiment and Kruks 30-31 The One impersonal perception

Butler here calls it an "investment," and it seems to be saying that MP adds an ahistoricity to the body, as a structural component, that runs counter to the reality, because the body is admittedly historical. We want to ask why MP does this, when he later has to take it back, or add historicity back on top. What does he gain? He is able to abstract and depersonalize the body itself, as a matter of structure. Kruks is fine with this, Butler will show it to be problematic, at least in his account of sexuality in PP.