The NORTON lay down gearbox was first used in the Model 7 Dommie Twins for 1949.
ES2s and Model 18s and 16H followed suit in 1950, Inters and eventually Manxes a bit later
And as mentioned above, was last used until somewhere midway through the 1956 model year.
It is pretty much the same inside as the previous upright Norton box - except that ALL the operating mechanism is inside, and lubricated.
If you want to get technical, it (and earlier Norton boxes) are faintly based on the old Sturmey Archer boxes.
Except that NOT ONE PART is actually the same as any Sturmey Archer box.
And, for a bit of trivia, it was actually built for Nortons by Burmans - Sturmey having ceased gearbox manufacture in the early 1930s.
Nortons bought the design from Sturmey back then, and came up with the dolls head, then upright, then lay down designs in turn...
Hopethishelps.