The comments below, refer to 135 film, not 120, however I believe that within reason, similar results would be obtained with 120 film.
I have been a big user of Neopan 400; Ive used around 300 rolls in the last couple of years. I have also used about 30 rolls of Neopan 1600 since the start of this year.
Fuji have a wonderful film in Neopan 400 and I found it works best for me with wonderful results and near grainless prints, when developed in my system using D76 1+1.
One of the features of Neopan 1600, according to Fuji, is that Neopan 1600 is designed to be developed at the same temperature, dilution and time as Neopan 400. Making it possible to develop both films in the same bath. From a photographers point of view, this is a great help in simplification and reduction of cost and time.
Basically this is true and I have developed Neopan 400 and Neopan 1600 in the same bath and had fantastic results, doing this.
I myself have found that Neopan 400 works best for me with my system and cameras when I expose it at 320 ASA. Using the same processing system, I have found that Neopan 1600 comes out at very close to 800 ASA.
When it comes to grain, you will notice only a small difference with normal cloudy bright weather, or broken up clouds and dappled sunlight, between the two films. If the light is of a high contrast, then you will notice the difference more in the contrast than in the grain, with Neopan 1600 being reasonably contrasty by comparison.
If you are shooting in low light, then the grain in Neopan 1600 becomes prominent. The same happens with Neopan 400, but the Neopan 1600 film will really have prominent grain, in comparison. We are not talking of golf ball sized grain, just prominent.
I believe that Neopan 1600 is really an 800 ASA film, this I assume from my own usage and from technical papers from Fuji.
I have a tendency to believe that somehow, you really do have Neopan 1600 film wrapped as Neopan 400.
It is not inconceivable; remember that there have been massive reductions in film manufacturing. This along with the possible movement of master rolls having been stored or moved around with new or replaced staff, anything is possible. I do realise that this is a highly unlikely possibility, but it would be possible nonetheless.
PE makes some interesting comments regarding the edge markings being of the incorrect exposure and that they would be noticeable. I agree, but as the difference in reality with the two films only being 1 stop or 1¼ stops at the most, not 2 and possibly a bit stops, then that could explain that.