DocScrutinizer (and others) made a study on this:

http://wiki.maemo.org/User:Joerg_rw/jrbme/

As for originality - I would say it's no big deal. As long as the charger can provide enough current and correct voltage (which yours clearly does), and has the short on D-/D+ pins (yours does too), then you're fine.

I would even bet most of the remaining N900 have not had the original charger treatment for a long time by now