Purpose of mercury switch in old doorbell
I have this exact Friedland chime. It has the number 280-860 on the back. Here's how to wire it: 

The terminals are numbered, clockwise from top left:
3, 0, 2, 1, 4.

The solenoid is internally connected to 3 and 4.

The upper 2 batteries go to 3 and 0.

The lower 2 batteries go to 2 and 4.

The mercury tilt-switch is connected to 1 and 3.

The front-door bell-push should be connected to 0 and 1. 

This gives a continuous DING-DONG, DING-DONG, ... as long as the bell-push is held down, by striking the vertical plates at either side. 

The back-door bell-push (if used) should be connected to 0 and 2. 

This gives a single DING when the bell-push is pressed down, and a single DONG when it is released. 

Operating with 4x 1.5V D-cells, that is all there is to it. 

If you want to use a 6 volt transformer instead, be sure NOT to fit batteries as well.

Link 2 to 3 with insulated wire (be sure to remove the batteries otherwise this will short out the lower pair).

Connect the 6V AC supply from the transformer to 3 and 0.

When fixing to the wall make sure the mercury switch is at the bottom, as per the photo above.

Job done.