The MX-18290 was not designed to fill any transmission security devices. It's only function is to fill a SINCGARS radio with the hop set and with the hopping key. MX-18290 is very similar to the KYK-13 in physical appearance. The cable which interconnected any fill device to the crypto unit acquired the sobriquet donkey dick.

The button in the center is to INITIATE a transfer into the fill device ( fill me) - either from a key generator or from another fill device. Typically, the initiate fill command is given from the device receiving the key. In some cases just connecting the fill cable starts the fill.

Moving the function switch to the ZA position and pressing the push switch would zerioze the contents of the MX18290 device.

Left view