Rules have changed since I did any new registering. I also just sold mine, so no longer have access to its V5C to check info. Its ironic that the only two driving, registered, road legal cars that were about, have both now been exported, as no one here saw fit to pay a premium that the foreign buyers recognised. Hey ho.
But if you have a chassis plate on your Comtesse it should state that the car is homologated in 1975. That means its accepted by the French as a car. That means the EU accept it as a car. Indeed this date ends up being the default date on the few that have been registered, just because its on the chassis plate. My one might have been later, we think. A study is under way to try and track the build, changes and dates of these cars through production, but unless owners chip in with all their information, it will not be a big enough sample to be acceptable as a dating resource. So its up to all the Comtesse owners to act in their own interests. This initative was successfully done for the Invalid Carriages, which can mostly now gain a registration. So it works.
If you have no chassis plate. Oh dear! As sans permis, these cars do not normally come with any usable French documentation. Be very hard to register.
Since the chassis plate states the information needed to be known by the clipboards, I see no reason why they need a letter. They only need to have the car inspected. That might be acceptable by picture and MOT on the chassis number. The MOT inspector acting as the inspector on the clipboards behalf. Otherwise you would be directed to go to them, so they can look at it, which might be a pain in the arse, depending on where you live.
Part of the problem seems to be that these cars do not neatly fit into British banding of car structures and types. They should be classed as moped, as mine was. It is, in effect, the same as an Ariel Three or a Honda Stream. But there seems to be signs of deflection to Quad status. I suspect this is the easy answer with lack of proof about weight etc.
I do not know who, if anyone, is dealing with registration of post Classic Cars. Mike Shepherd is the RUMster who masterminds number reclamation, and re registration, but new registration is a different thing altogether.
I know this is not much help. There should be folk out there who have been through the latest series of nonsense needed to register a car. It sounds rather tedious, so I am afraid I stopped concentrating.
Saxonette is a good little unit. Though I have no reason to say the Motobecane version is inferior. I had to pull my engine half apart so should there be an issue, might be able to offer advice.
Stuart Cyphus is collecting information (The same might appear in RUMcar news one of these days). However I do not feel I can pass on his new private email right now, as he is rather preoccupied with personnel issues. You can try a PM. I will mention this thread when I chat to him next.