What kind of "safe" was carried on the Space Shuttles?

Question

This Smithsonian/Air & Space Magazine article about the secret/classified Space Shuttle missions contains this anecdote:

On the seventh day of the mission, Mattingly and pilot Hank Hartsfield were getting ready to return to Earth and had just stored the classified checklists in Columbia’s safe. Sunnyvale then asked them to perform “Tab Echo.” The astronauts looked at each other; neither could remember what Tab Echo was. They opened the safe, removed the checklist, and began paging through it. Sure enough, there was Tab Echo: “Store checklist.”

A "safe" on the ground is usually a heavy steel thing, not something suitable for putting into orbit. It seems strange that a spacecraft would bother with any kind of "safe" at all unless someone thought Moonraker was a documentary; although I would guess with the concept of "need to know", not all the astronauts on board necessarily needed access to the classified checklists, and it could be helpful to restrict access by the various ground crew.

I can't find any other references to a safe on the Shuttle; of course I can find lots of info about safety and about computers going into "safe mode". It may also just be that this information is still not publicly available.

1. What was the "safe" like that was used for storing classified materials on Space Shuttle missions? A strong steel box like we usually think of, or just a lightweight thing to keep honest people out?
2. Was a safe carried on all Shuttle flights (something permanently installed), or was it removed for non-secret missions where it was just dead weight?
3. Did Endeavour get provisions for a safe (or other related equipment) for possible DoD flights (which I don't think it ever flew?), or was it only equipped for civilian missions?

Answer 1

It was just a middeck locker with a padlock on it.

We had one drawer, one locker that was where we kept all the classified material, and it was padlocked. So once we got on orbit, there was nobody going to steal it because we didn't have to worry about it. We unlocked it and did what we had to.

Henry Hartsfield oral history

(Photo of middeck lockers on orbit)

(Source: NASA)

(Schematic of middeck lockers location )

(Source: Did the astronauts seated on the space shuttle mid-deck have responsibilities during reentry and landing?)

Since the middeck is stowed considerably prior to launch, presumably the "safe" was to keep uncleared technicians working in the middeck prior to launch out of the documentation.

Other classified missions could also have padlocked a locker, if required.