How does this Chubb Detector Lock operate?

[lorddlm2]

Hi there

I have a Chubb detector lock with its original key, and am very interested to know how this particular version would have prevented and indicated illegal access attempts. I have attached a few images of the actual lock, along with a borrowed image (found on this forum) to show a part of the lock mechanism that I didn't photograph myself.

The lock itself has the serial number 1674333, and I am assuming that it dates to circa 1870?

Im not sure of the terminology, but on the reverse of the lock frontage is a removable disc that has a 'roofed' area where key enters it. This disc has a small ridge in it that engages with a mounted spring.

If anyone can explain how detector part of this lock operates, and what the correct terminology for the components are, I would be very grateful indeed.