Parautoptic wroteThe purpose of these was usually to allow a container with a less secure lock to be used for confidential material.
More for the purpose of making surreptitious access more difficult than for foiling a brute force attack.
I’m at differerence with you on this Parautoptic. As I see it the closing of the keyway by a shutter was part of the original specification and solely intended to make the safe resistant to explosive attack, firstly gunpowder then high explosives. The Chatwood Treasury was particularly effective against this latter form of attack.
On the other hand Dual Control was originally known as Partnership locking which adequately describes the system and which would be incorporated n the original design specification.
The earliest wrought iron security cupboards as they were know before fire-proofing were
usually fastened by wheel locks and box-of wards locks which with their large keyways were very vulnerable to gunpowder attack.
The only defence against this was the lockable escutcheon with a compact Barron lock in a casing with chamfered edges to resist sledging. This by design could also be described as dual locking but not necessary the intention.
Later in the century brass escutcheon locks were commonly used incorporating very small lever locks. The application was probably as a retro-fit on single lock safes where dual control was later considered desirable against acts of collusion.
Dual control was also available within a single lock of course.
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