There are two different models of Hornby-Jouef cereal hoppers: one with flat walls, the oldest, which I believe originates from Rivarossi; the other with curved walls. The first is too high by 0.8 mm, the second is too low by 1 mm! This would not be too inconvenient if these wagons were not running together in a train, where the difference can be easily seen, both at the roof and at the buffer level.
About mechanics, we see that there is no three-point suspension, but a four-point one! This is obviously unfavourable to running stability, since the bogies have no backlash with respect to each other.
Referring to the NEM 302 recommendation, these wagons are too heavy: the curved-wall model is 94 g for a recommended mass of 70 to 91 g; for the flat-wall model, it is hardly better with a mass of 89 g. For amateurs of long realistic trains, this represents an undesirable excess load for the locomotive when the layout presents ramps.
The “flat-walls” is easy to open (clips at the top of the vertical faces). Its ballast consists of two steel plates of 51 × 26 × 1.5 mm, 13.5 g each, superimposed and secured by two screws. Simply remove one of the plates to obtain a mass more in conformity with the NEM. Unfortunately, the “curved-walls” appears to be more difficult to disassemble, and I did not insist, for fear of deteriorating. And I did not find an exploded view or advice on forums that could have helped me.