The RD is a Simplex SX 610 GT/SP with a 34T large freewheel sprocket capacity. They shifted about as well as pre-SIS Shimano and Suntour friction RDs.
Not shifting to the small sprocket can mean the derailleur needs to be thoroughly cleaned and lubricated, same with the cable and housing. A worn upper pulley can cause shifting problems too. Also check the upper and lower limit screws. Clean and lube the chain too.
I have a Simplex SX 610 GT/SP on my 1980 Moto Grand Jubile (still called Grand Jubile in 1980).
I've never seen a Motobecane with Simplex dropouts. They used Campagnolo, Huret and Suntour dropouts with integral derailleurs hangers on their mid range to top quality models.
Per @
T-Mar in message 8, the dropouts on your bike were made by Huret. Motobecane used them on a lot of their models from ~1978 through the early 1980's. They appear to have been made of investment cast steel.
The derailleur hangers on most Huret dropouts from the mid 60's until the mid 70's were a proprietary design that only worked with Huret derailleurs.
By the mid 70's Huret gave up their fantasy of world domination and changed their derailleurs and dropouts to be compatible with the de facto standard Campagnolo design.
You can mount most brands of 1970's on C&V derailleurs on your bike without any problem except for early 70's Hurets which require an adapter.
That's not a washer on your Simplex RD. It's the travel limit stop that's part of the derailleur. There's several different styles.
A word of caution, don't try to remove Simplex derailleurs by removing the nylon plug in the hex opening in the pivot bolts. Those hex openings are just for adjusting spring tension.
Loosen the mounting screw on the inside of the dropout.
Simplex and Huret (later called Sachs-Huret).
verktyg
Chas.