It's a very interesting beastie, but nothing (I think) to do with FN. The M12 motorcycle was made with a driven-wheel sidecar 1937-1940, and the Model 12 T3 Tricar was developed along-side, and built in reasonably large numbers. In the FN Tricar, the front wheel is at the centre of the triangle, and the rear differential was offset to the right, and the chassis detail is quite different. I have a book about FN with an extensive chapter on its military machines, and there is nothing to resemble the "monstrosity".
The "monstrosity" is neither "driven sidecar", nor "conventional tricar" - the front wheel is neither in line with one of the back wheels, nor equally-spaced between the rear wheels. This should be an id feature, as should the braced telescopic fork.
It's such a weird thing it's hard to even guess the date, but I suppose somewhere between 1938 and 1960?
Fascinating.
Leon