A cylindrical mechanical combination lock, metallic grey with a black dial, separate from a metallic cylindrical sleeve.
A different side view of the lock, showing the rear in darker metal with some kind of mounting flange.
The same exhibit, but showing the placard with text describing the exhibited item:
C'mon, Be a PAL
Permissive Action Links, or PALS, are devices that are intended to prevent the unauthorized use of nuclear weapons. If anyone who didn't have the code got their hands on one of America's nukes (like a terrorist or an enemy force), the PAL would keep them from being able to detonate the weapon. The combination of the lock itself was split between two members of the weapons team to prevent any one person from gaining access.
Permissive Action Link, US, Ca. 1960s-1980s
The NSA is the only intelligence agency with an unclassified museum open to the public. They had on display a mechanical PAL, or Permissive Action Link, that would protect US nuclear weapons from unauthorized use. It seems a mechanical combination lock protected some nukes through the 1980's!