Ventus 2C W&B - 15M vs 18M

My V2C came with a factory W&B (for 18M) and a meticulous spreadsheet
from the first owner to track changes (like substitution of a brass
tail wheel, or using factory fin weights). I'm going over this
spreadsheet in detail prior to getting my own W&B done but am unable to
figure out how the 15M configuration is supposed to be accounted for.
(There's nothing in my manual about this.)

The manual says the empty CG must fall between 585 and 626mm aft of the
datum.

The factory W&B does list a 15M weight that is 12kg less than the 18M.
But looking at the planform from one of the dealer web pages, it looks
like the center of mass of the outer panels is about 0.4 meters aft of
the datum. Now, this is very close to the nominal in-flight CG, but If
I add a moment arm adjustment of -4800 (400mm X 12kg) to the
spreadsheet, the empty CG comes out at 640mm, aft of the limit.

So it's not clear to me how or if I should be accounting for the 12kg
difference in the two weights.

Monty at M&H has forwarded my question to the factory but I'm
wondering if anyone else has dealt with this yet.

~ted/2NO