Actually, I just had the beast out! My first attempt was with
a 19mm box. Didn't fit. A 17 box seemed to fit but was too long
to allow any torque to be applied.

Used a small pipe wrench instead. It unscrewed right smartly.

The vent looks dirty on the outside but clean on the inside.
Turns out that it -is- a 17 after all.

There's a little felt disc inside the vent top. PartsShop guys,
is this a regular field service item? Is there a part number?

The one thing I would have done differently here was to
have used something tougher (nozzle of a pressure washer?)
to clean the top deck of the diff before removing the vent.

I'm concerned about gritty particulates falling in, and I did
clean first, but apparently not enough. A shop rag wipe on
the diff top with the vent removed showed lots of crud there.
Dr. Murphy says that some of it probably went down the
threaded tap for the vent.

What I think I'll do now is clean and replace the vent
(and felt disc?), then run two quick successive changes with
el cheapo generic gear oil to flush out any crud that fell in
during the vent service. Should suffice, yes?

Then I'll do one more change with Mobil 1 gear oil, and
put this to bed for now.

Ron H