Sargent & Greenleaf 833 High Security Padlock (disassembled)
The lock is able to resist many attack vectors, such as picking, bumping, wedging, cutting, sawing and drilling (it features ceramic discs inserted inside the shrouded shackle). It is even supposed to be able to withstand liquid nitrogen attacks, explosives.
In case of key compromise, the six pin cylinder can be re-keyed by using the operator key to disassemble the lock and exchange the cylinder core.
This type of lock was usually found in high-risk locations such as bunkers, weapons and munitions depots.