Correct Phil.
They lifted a pic of an 18.
The cycle selector buttons on the 17 and 18 were the same. It is Action Indicator lights that are different and the door latch handle changed on the 18-20 series. I believe the door latches were the same on the 16 and 17 series.
And the 15 series was the major design change to the HydroSweep from Big Blue the old Cast Iron arms, new racking configuration and also from the start of the 15 series the gravity drain was discontinued.