The Hexagon vehicle that has been put on public display must be the "hangar queen," the qualification vehicle that was put through punishing vibro-acoustic and thermal-vacuum testing environments at factors above the expected launch and on-orbit environments to qualify the design. Since it was intended as a non-flight item, the idea was to "bend" the vehicle, i.e. stress it the point of being unreliable for flight, but that was OK as long as the qualification testing didn't "break" it, i.e cause a serious failure. The 20 flight vehicles were acceptance tested to expected vibro-acoustic and thermal-vacuum levels to catch workmanship defects. We called this "shake and bake."
It is reported that a film bucket from the first KH-9 Hexagon sank to the bottom of the Pacific Ocean in spring 1972 after Air Force recovery aircraft failed to snag the bucket's parachute. My recollection is that another bucket went into the drink but was recovered shortly thereafter, and there may have been two such events. The buckets were equipped with "sea plugs" made of an alloy that would react with seawater and dissolve in about 24 hours, sinking the bucket; thus, if the Air Force couldn't find it by then, at least it wouldn't fall into the wrong hands.
Some of the recovered buckets were refurbished and flew two or three flights. These constituted the original fleet of reusable spacecraft, beating the Space Shuttle fleet to that distinction by a decade.
A lot of the Byeman names that one finds in open sources do not seem to have any reason to them. But with regard to Hexagon, my theory is that it suggests the shape of the cross section of the forward section: it is like a square with diagonal cuts across two adjacent corners, making six sides in all.