Without further ado here is an "infographic" of the lockworks of a Rizzini BR-550E.
The design is typical Val Trompia trigger-plate with the addition of a hammer blocking wedge
that prevents firing if the sears fall off while the safety is on (as if you dropped the gun).
The function is similar to a best gun side-lock sear blocking safety but this design is much simpler and easier to manufacture.
My Beretta double 12 ga. have no such feature at all.

1-Safety on, hammers cocked

2-Safety on, right sear pushed off, hammer blocked

3-Safety off, right sear engaged

4-Safety off, trigger pulled, right barrel fired

5-Right barrel fired, safety back on, sears dropped as with firing inertia (recoil)
Hammer block no longer effective for left side.

6-Right barrel fired before recoil

7-Right barrel fired after recoil

8-Left barrel fired

9-Left barrel fired, safety back on

I'll have more on regulating some non-factory rounds in the next installment.