Looks like a cheap somewhat flimsy small metal file cabinet. Not something sturdy or with a sealed lid. I bet if you buried that thing you may not want what would be left of whatever they put inside. Might look like that Belvedere!
And Bill not just the basement, that building/garage on the north side near gate 9A also has cars and tall shelf after tall shelf of stuff, heard there are other buildings as well. So I suppose it could be "buried" somewhere so-to-speak, but I'd bet on "publicity stunt."

Apparently it will most likely forever remain a mystery.
It is not in the museum basement and the grounds/structures have all been changed extensively since then.

Sadly odds are anyone who might have knowledge about its existence or location is no longer with us.

Are there no other time capsules from other years?

As if any other year really matters?!?!?!?!?
I don't know.
Nothing shows up in a quick internet search, not even '67.

bump