The designation "C.310" does not appear anywhere, not even in the very thorough Docavia book. We can't just throw hypothetical designations like that without a semblance of evidence. When you do, there is always a risk of your post being misinterpreted as fact. Do you have a particular reason to associate this project (itself hypothetical) with that designation? Does it fit the chronology?
Of course I know that very well,but that's only a speculative designation,I
never said it's a real one !,and I have my source,which explained that the
two designers had been developed many concepts just before and when
they joined Caudron,their inventions get the Caudron series after that,and
I always speculate to be nearly the truth,it's not just idea inside my head ?.
I have two sources,one I can give you,
Omnium Metallurgique Industriel
From 1931 to 1932, Riffard worked at Omnium Métallurgique Industriel (OMI), an engine builder whose products for motorcycles and aircraft were sold under the Chaise name. At OMI, Riffard was tasked with testing their first 95 hp (71 kW) inverted cylinder aircraft engine.[2] It was also during his time at OMI that Riffard designed an aircraft called the MR-300 intended specifically to compete in the Coupe Deutsch de la Meurthe. The aircraft's side profile was wing-shaped, and may have acted as a lifting body. While one reference says that it was powered by an 8 L (488 cu in) flat-sixteen engine, others say that it was a Chaise flat-8 designated 8C, producing 300 hp (224 kW).
Also at OMI at this time was Russian aeronautical engineer Georges Otfinovsky, with whom Riffard would work on several designs later at Caudron.
en.wikipedia.org