Now the mystery is solved: the two large vertical spools at the front are for carbon ribbon. Indicated on the spool in green is the feed-take mechanism and ribbon path. The Hermes Ambassador has two sets of ribbon vibrator: one for the regular nylon ribbon, the other one carbon. Correspondingly its color switcher has four positions, carbon, black, read and stencil.
In 2014, renowned Australian typewriter collector Mr. Robert Messenger posted a video on his blogspot, demonstrating how this sophisticated two-ribbon mechanism works. It's very interesting to see.
I've attached the video in this post. Though in the first part of it, Mr. Messenger introduces the de-jam feature of Ambassador, which, to me, is not a novel idea as all Chinese Flying Fish desktops come with a de-jam key and I've seen a lot. Just in case you are curious about how Chinese typewriters look like，please refer to my previous post of which a link is provided.
You may fast-forward Mr.Messenger's video to the latter part if you are more interested in its carbon ribbon mechanism.
Now an obvious question,why bother to have two sets of vibrator? Is carbon ribbon superior to regular ones? Is it to compete with the electronic typewriters already popular when the Ambassador was launched in 1970s?
Sorry for being so inquisitive.