I failed to be general, what is the ratio between the radius and the line length, you may want to show an example of your intersection.

Yes I definitely recognise the system, and also have the confusion on the 4th symbol you mention. Below is the Windows XP shutdown screen which features the 3rd and 4th symbols:

I've always assumed that's a binary 1 for "on" and "0" [zero] for off and the standby and shutdown icons combine the two symbols in different ways.

I think "I/O switch" is the technical name for the on/off switch, which I believe predaties computers, going back to the early days of electronics. Either the switch is on or off, directing or stopping the flow of current. (This doesn't address the fact that if the thing is unplugged, its state is actually mu, being neither.)

This is indeed related to the binary concept. I think elliot100 is right about the 'binary 1 for “on” and “0” [zero] for off'

Pretty much all the Macs I've used have the circle halfway intersected by the vertical bar on the on/off button.

This Berkeley study might be of interest.