2000 Odyssey- engine bolt id? coolant question

I have a 2000 Odyssey with a 3.5l VTEC engine. If you get under the car
and look up on the side of the engine block facing the front, there is a
larger sized flanged bolt with an aluminum washer under it, much like
the filler bolt on the auto transmission.

If I had to guess, I would say this is a coolant drain for that side of
the block, the drain bolt ID'ed in the owners manual is on the other
side of the block.

Does anyone know for sure what this bolt is?

Why do I care? The Honda dealer said that draining the radiator and the
engine as shown in the manual would still leave a good bit of coolant
in the engine. I would like to flush everything with water, but if I
can't get all the water out, when I refill with the Honda 50/50 premix
it won't be 50/50 anymore. The (type II) premix is the only Honda
coolant the dealers around here carry now.

Thanks!