Series 20 at the newest from what I can make out.

Pix are terrible so I won't post more than the link here.

That's a single wash-arm, 17 series.  Cool machine.  Gorgeous top.  We used one at the 2005 wash-in in Tucson to do the dishes, with Bob at the helm for maximum efficient loading.

Yep, Nate beat me to it.  I was gonna say 17.  I don't think this style of portable made it into the 18 series at all, only portables then didn't have a control panel on the back top.  One of the 1-level wawsh wonders.  and thanks Nate, I did do pretty good with that loading.  But then again, all it was wre plats, glasses, and serving bowls lol.  Nothing unusual for this DISHwasher. 

It just goes to show, you can't judge a machine by its buttons.  They looked later than 17 to me, but I guess not all models of a given series had buttons that were the same shape.