Here's the definition from 18 USC 921 :
Getting BATF to accept such, and not penalizing the FFL next inspection, is the issue. Generally, the FFL's question will be something like 'will this lose me my license?' and, if in his/her opinion the answer is 'yes', that's too much risk.
The re-used lock from a prior purchase should satisfy.
(34) The term “secure gun storage or safety device” means—
(A) a device that, when installed on a firearm, is designed to
prevent the firearm from being operated without first
deactivating the device;
(B) a device incorporated into the design of the firearm that is
designed to prevent the operation of the firearm by anyone not
having access to the device; or
(C) a safe, gun safe, gun case, lock box, or other device
that is designed to be or can be used to store a firearm and
that is designed to be unlocked only by means of a key, a
combination, or other similar means.
(A) a device that, when installed on a firearm, is designed to
prevent the firearm from being operated without first
deactivating the device;
(B) a device incorporated into the design of the firearm that is
designed to prevent the operation of the firearm by anyone not
having access to the device; or
(C) a safe, gun safe, gun case, lock box, or other device
that is designed to be or can be used to store a firearm and
that is designed to be unlocked only by means of a key, a
combination, or other similar means.
The re-used lock from a prior purchase should satisfy.

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