Below is an example of the Sebastian Pocket Cash Register, also from the collection of Robert Otnes.
Pocket Register of Charles Sebastian
Patented 1898 (source Robert Otnes with permission)
Patent Drawing for Pocket Register of Charles Sebastian
Patented April 26, 1898 (source Google Patents)
I haven't tracked down a patent for the American Cash Register Co. piece pictured at top, but I have found these others:
Several patents for Pocket Cash Register (source Google Patents)
The idea of these pocket registers certainly seems to have been fermenting in the late 1890s, but the devices do not seem to have generated many sales. These items are rare.