Andrew,

You said,
  'H doesn't fit, and as others have pointed-out, doesn't discriminate
between center-crank (boxer)
or 2-stroke outboard dual cranks.'

The standard, widely-used 'H' designation stands for a
'horizontally-opposed' cylinder arrangement.  It does describe this exactly,
I see no vagueness there.  These are very common and offered by several
manufacturers from early cars to present-day.........

Does a 2-stroke outboard dual crank engine have horizontally-opposed
cylinders also?  I've never even heard of one, let alone on a common
automotive production vehicle.

Mike B.