('SESSION', id: 10285673650225743670, token: 75320603A0aaJWubl-Y, contents: [])

Now, it happens that the 'token' field and the browser id have the same value (75320603A0aaJWubl-Y), so I wonder why should there be the 'token' field at all: isn't it redundant? On the other hand, if "browser id" and "token" are conceptually to be kept distinguished, why do they have the >same value? And, again, what's the 'token' field intended for?

The "token" is the browser id. The "id" is the session id. The __repr__ of a session data object prints out the browser id as "token" for "hysterical raisins". Printing the value of "token" is not redundant, it's just misnamed. It should really label it "browser id". This could be considered a bug if you're super-picky. ;-)

??? If this is so (i.e. if the session id is not stored or seen on the client in any way), what about the id: 10285673650225743670 key-value pair contained in the cookie sent to the client browser? Please, explain further on.

There is no such key value pair in the actual cookie sent to the browser, you're mistaken. The cookie only contains the browser id. You can check this if you like by examining a browser id cookie on your filesystem. HTH, - C