While researching the Astro I found a picture of the Mercury astronauts in their desert training. Note the Astro strapped to the leg of Gus Grissom (sitting middle)and Alan Shepard (sitting right), you get a glimpse of the handle on the side of the astronaut sitting on the left.
The following photo if of the door going on Alan Shepard's Mercury capsule.
So close to seeing his Randall taped to the inside of the hatch!
This photo is of the inside of a Mercury hatch. No slots, so more than likely the knife was taped on.