When I bought a new Phantom Supreme 10" in March 2013, I had the same problem with a Koetsu Onyx Platinum that - for some reason - is 2 or 3 grams heavier than every other stone Koetsu I’ve encountered (very thick walls). No extra weight or 2nd counterweight was included. In fact, despite ordering brand new "A" stock via an authorized dealer, my package was even missing the manual. Perhaps the 12" wand could pose the same balancing problem for the lighter wood body Koetsus.
The solution proposed to me at the time was a supplemental disc weight that can clamp down onto (piggy-back style) the knurled adjustment thumb screw for the main counterweight. I was offered it for $100 from Graham, but Musical Surroundings (their distributor at the time) happily sent theirs out to me fast and for free. It works great with my heavy Koetsu, but can’t balance (in the other direction) the lighter ones.
It’s a shame Musical Surroundings is no longer the distributor, because they could often step in promptly when other measures failed.
Anyways, I recommend you pick up a clean used Fidelity Research FR64fx or 64S tonearm to run your Koetsu. It costs less than a Graham, sounds much better with Koetsu, and good headshells (e.g. the Ikdea shells) are far cheaper than Graham wands.