I didn’t find a photo of a MS20, but found this video of a 1940 MS15,
which I assume is similar.
You’re correct, the MS20 should be a single cylinder.
If you found MS20 on the rope pulley, I’d say someone put
that pulley on your two cylinder engine.
Can you email the photo to one of us so we can post?
https://youtu.be/raBR6RaGtU8