Burakkucat said to me in a PM that he was doubtful that all my modems have the same supervisor password.
The answer is that they most certainly do share one single supervisor password. I know because I have it set the password as a saved text-snippet/macro in my telnet client app (iOS Prompt2 app) so I can just inject the password into the telnet session with two clicks and I use that when logging in with telnet. I have one macro for the admin password and one for the supervisor password - I suppose I could have made them the same now I come to think of it.
Is the syntax of a supervisor password limited? I presume not, not the test would be if you attempt to set it to something random and strong yourself.
I just don’t understand those passwords. They could come from some random number generator that returns a 32-bit iirc integer and converts that to ascii hex. They could mix in the MAC address of the device as the random number generator might need more sources of variability, or also mix in the current time, hi-res if possible. If they did any of those or similar things then the default passwords would never be the same across my modems. However that presumably is irrelevant as I’m not looking at virgin state / post factory reset state modems, I’m looking at the state after loading up my standard saved config and I just presume that supervisor is one of the account name-password entries stored in the xml config, just no different to "admin".