WP PORTABLE DW
The model # designates the model year, sKp 55 the K means that it is a 1963 model, it could have been made in late 1962 as appliances like cars usually came out a bit early as everyone was trying to get the jump on the competition in those days.
It is really cool to see this machine with the SS upper wash-arm, this may have been WPs first DW with a upper wash-arm. The next generation of WP DW had an all new combined circulation and drain pump, these models had a large round white plastic filter in the bottom. On these machines the upper wash-arm looked exactly like yours except it was pink plastic.