The ruler in the photo isn't close enough to the bulletmold to allow more than an approximate estimate of size, but it indicates the bulletmold would produce a bullet which is a little bit larger than one centimeter in diameter. So, I agree with NOLA Ken, it is probably an early version of bulletmold for the Model 1873 Winchester .44-40 Rifle.
To see photos of a lengthy timeline of cartridges for that rifle, go here:
THE CARTRIDGE COLLECTOR