I noticed that the 1984 Motobecane catalogue shows the SLJ 6600 and SJA 102 derailleurs on their top end racing bike, the Team Champion. The derailleurs shown in the catalogue photos actually feature the same tri-color "M" logo which was used by Moto at least during 1979 to 1984, maybe earlier. The derailleurs you guys have look a wee bit earlier than the '84 versions (those rear derailleurs had dark plastic pivot bolt caps).

The design of the 6600, with its short pulley cage hanging low off the bottom pivot bolt, made it best for largest freewheel cogs up to only around 24 teeth and a total chainwrap capacity of around 22 teeth - which was certainly fine for racing bikes. And the dropped parallelogram design (pretty much like most any modern derailleurs today) made shifting across cogs of say 13-24t very clean and efficient.

Sorry about this grainy image ... original catalogue photo was only 30 mm x 38 mm.

[This bike was fitted with a 13-18 6-speed freewheel and 42 x 52 chainrings]