last update: December 2004
The lock has the normal operation for a combination lock. The combination has four numbers and is always dialed in the same way;
Although there are four numbers in the combination, only three are significant and settable. The fourth is fixed and will always be 0 for a right-swinging lock, or 25 for a top-swinging door like this.
This is the rear of the lock, when open, but with the bolt returned to the "locked" position. Once unlocked, the bolt can be moved back and forth repeatedly, by moving the dial about 1/4 of a turn.
Note the "broad arrow" marking, typical for UK government issue. The rear case is held on by two screws. Normally these would be lock-wired and sealed with a lead crimp when in service.
Later versions have a "hook" inside the case that prevents it being removed unless the lock is unlocked first, even if the screws are undone.
Setting the combination uses a key, inserted through the small covered flap on the rear cover.
Not all combinations are permitted.
Most of the lock's operation centers on the cylindrical disk pack in the lower half of the lock. This is driven by the dial, but revolves backwards relative to it.
The disks carrying the combination are invisible beneath the outer disk. The first number is on the most deeply hidden disk. If the safe is open, but the lock combination is unknown, it is possible to open the lock by inspection of these disks with a small torch.
When the combination is entered, the notches in the disks line up and allow the comb to fall into the notch. The final half-turn rotates the outer disk, so that the visible notch in it catches the large lever and pulls it down, opening the bolt.
Several aspects of the mechanism are notably sophisticated. There are even a pair of lead shields to prevent X-ray examination.