The number is a replacement for when it was first registered; some years back they were less stringent than they are now. That is why its non transferable.
It did not have to be a lawn mower, villiers engines and their french knock offs were used in all sorts of machinery and yes I have seen a two exhaust lawn mower.
its got the stubby lever because its not its original box, its got a K/s but it probably had cycle type pedals when it was made, its what the french classed as a BMA.That gear-change is so obviously home made.
I think you need to do a lot more research into old french utility bikes from the 1930s, you can talk it up as much as you like, its a lash up, never been on a track unless it was a cycle track.
It is what is and if you like it all well and good.
I've been in bikes dealing and import/exporting them and have seen so much of this.
Have a holiday in france and do the rounds of the Brocantes/Vide Greniers and Bourses[auto jumbles] and see lots like this.
Sorry to be harsh, thats my lot.
nearest I can find to your thing is this
http://motoensavoie.canalblog.com/archives/2013/05/30/27284801.html