Andy you did it again, congratulations! I'm glad I could create a difficult one, even for you
FN is a marque with great history and they were the leaders in their time. There is a resemblance with BSA (Birmingham Small Arms) as both firms were producing guns and rifles and gained a lot of knowledge in metallurgy and their tooling provided great precision and tolerance accuracy to their products.
The FN rifles were very popular worldwide.
It seems they were a bit overambitious about the fork they installed on this bike, and they had to withdraw it soon after (see attached pic). I read about this bike in a magazine, Sammy Miller restored an example of this bike and let the writer ride it.
Although theoreticaly this front end would solve a few problems of the contemporary forks (girders mainly) it proved to be a bit problematic. Even on the perfectly restored bike, there was vagueness and the feel was really strange. It was very comfortable on the straights which was very useful with the roads they had back then, but once the bike was not upright the fork couldn't handle the cornering forces.
The condition was even worse in reality as there are a lot of bushes and shims in the linkages, and as a result play develops fast and 'vagueness' becomes, 'it goes where it wants'.
The later version is less compllicated with much fewer linkages but still it wasn't adequate and had to be replaced by a conventional fork later.
If you watch the pictures carefully you will see that there is a rubber rear suspension also which remained for all the models. At the time very few bikes had rear suspension, and the y were of the plunger type providing little comfort.
The rubber controlling the rear end can be seen under the saddle, where it is attached to the cantilever and swingarm.
Here you can see the same bike with a normal fork.
I'm glad you enjoyed it