I've just discovered that it's simply ASCII encoded in hex, near the end of *.swb file - for example, 'password' is '70617373776f7264'.
It would be nice if future versions (if any development is still being done on SwOS) could use a proper hash function like Unix crypt().
Until then, be careful not to leak the config files, and better use a separate password (not used anywhere else) for RB250GS switches.

I am pretty sure you can also get the password out of routerboard backup files with a certain tool. I thought I had saw something about this somewhere but I can't seem to find it again.

I guess that is a good thing

Keep your backup files safe, and don't leave them in your router