I am para-phrasing from Bob's book but on the first "2" sub-orbital flights, Alan Shepard on May 5,1961 and Gus Grissom's on July 21,1961, the Randall Astros were stowed in the hatch of the space capsule. Both men carried NASA knives. verses their personal ones.
Beginning with John Glenn's orbital flight (Feb. 20, 1962) and from that point thereafter the knives were housed in a special sheathed compartment on Mercury's survival kit.
Scott Carpenter's "3" orbit flight also resulted in his knife being lost and it "was" his personal knife. That flight was on May 24,1962. It is not clear from his letter & request for a replacement knife to Bo if the knife was lost at splashdown or sometime afterward. The knife was lost by the time the capsule was returned to the mainland.
I only hope that one of our more computer literate forum members could do a search and post a photo of the Mercury survival kit. You can clearly see the "Astro" attached to that kit. Hope this helps.
Best, Capt. Chris