A few pics for you JP:

Chrome one is a 84 while the white one is more 85. (Motobecane switched name to MBK around this time, in a joint venture with Yamaha, hence the new brand on the white frame).
It was later declined in the MX500 available in different colors, but the 400 was only available in chrome or white
The main thing on this frame except the new semi twin toptube design was the welding process called "inexternal", invisible!!
Original french ad from 84

You see yours is pretty much a survivor, bars were CW copies called "espace" bars as for the Tuf Neck style seatpost.
Pedals were Shimano SX, not too sure about the stem..
PM me if you need some parts for it, might be able to help.