It is supposed to be a 45/70 twin. Marlin put the belt on it so a 450 could not be put into a 45/70 chamber. This took away the possibility of putting the bullet into an older rifle. This allows the factory loadings to all be loaded to the higher pressure limits that are set for the new chambering. The thought is that with a full capability new 45/70 rifle, such as the ruger #1, you could load the 45/70 equally with a 450.
I think the lack of demand for this chambering may also be a nod to the 45/70 being a nostalgic cartridge. While there is no nostalgia with a new chambering and it must make its way in the market on functionality only. I know that I just got a 45/70 because of nostalgia, not functional need.