Two-Key Cabinet/Door Latch; What's It Called?

[wlwhittier]

The body is about 3" X 2" x 3/4", Brass. I have three of them, essentially identical and keyed-alike. Two of the larger keys are steel, with cast-on zinc(?) handles; the three smaller keys are steel.

The large key enters first through the door or panel, piloting on the hole in the body on the latch face. The body is secured to the door on the side away from entry, as the bolt and countersunk attachment tabs indicate. A half-turn CW withdraws the bolt, allowing the door to be opened toward the large key.

BUT: in order to withdraw the large key, the small key must be inserted from the face of the latch (inside the secured space) and turned a quarter-turn CCW, thereby allowing the bolt to return to at-rest position, and freeing the large key (which has been captive) to be fully rotated CCW and removed. At no time is the small key captive.

My apology for inappropriate terminology...I am not very familiar with locks and their components.

Anyone familiar with this odd (to me) arrangement? What use/purpose was this designed to satisfy?

Thanks for lookin'...wlw