Sorry i didn't update this.
I found the other 8 mm recessed hex plug on the other side of the engine. Both of them are towards the rear of the engine. If you look below the two rear spark plugs on each side, they can be spotted.
I got about 5 1/2 gallons total out of the radiator and the two drains.
The 17 mm may also be a drain but since I got out what I understand to be the right amount of coolant I didn't remove it.
The coolant looked clean but after five years the additives must have been at the end of their life.
On another forum a poster explained how to open the radiator drain fitting using a 1/4 inch ratchet wrench and an extension. That was very helpful because it would be easy to damage the fitting. The square end of the extension fits into a square in the fitting. It only turns about one half turn CCW to be fully open.