The serial number of the Alcomonitor CC is on the back panel  OR   You can press the #3 button and the display will show you the serial number 

The Epson slip printer (light beige color) has the serial
number on the side of the device.

The Star slip printer (dark graphite color) has its serial
number on the back.  You will find the sticker pictured
below on the underside of the printer.