[onelifecrisis]

Originally Posted by ichso

So, to get this straight: All the three available buttons are set to a fixed ItemID and to a fixed behaviour. I suspect so because there are no hints what those buttons exactly do, so it must be hardcoded somehow.

Yes the IDs are fixed and the behaviour is hardcoded.

Let's take the 25m-260m switchable depth gauge fo an example. What I assume is that in stock SH3 (and MFD also) the 260 version gets printed first and the 25 version afterwards. So the only dial you see in the game is the 25 version.
Then one button refers to the 25m dial and toggles it's visibility. So, when you click it the 25-dial gets invisible and you then see the 260-dial (and the according pointer+value).
Is this right ?

Not quite. The buttons toggle specific menu groups on and off. Each button has two associated menu groups (whos IDs are also fixed and hardcoded). Taking one button as an example, its first menu group (containing, say, the 25m dial) starts visible, and the other (containing the 260m dial) starts invisible. Pressing it switches the visibility of both groups.

Answers above in blue.