@chelseadave
I have one Koetsu Onyx that is thicker, and a couple grams heavier, than my other stone bodies (despite all being spec'd at 13 grams) - this one exceeds the capabilities of my Phantom Supreme's stock weight. There's an extra, smaller counterweight that can piggyback onto the main counterweight adjustment's thumbscrew mechanism. It's not included in the box, but it probably should be (for that matter, my Phantom didn't even ship with a copy of the manual!). With the extra weight, you should easily be able to accommodate 20g.
Yeah, getting in touch with Graham - good luck. He'll probably turn up at some point and blame his email servers again. GET A FACEBOOK PAGE, BOB. Great, superb arms (I truly love the Phantom and enjoy it many hours each week with a rotation of a few Koetsu carts), but frustrating customer support. Back when I went through this, Musical Surroundings was a Graham distributor - M.S. has STELLAR service, and they supplied me a manual and the extra counterweight (Graham wanted 100 bucks for it), free of charge, in short order. I also wanted some extra wands that were perpetually out of stock, and eventually met my needs on the used marker. A couple years later I noticed M.S. stopped distributing Graham...I'm sad about that but don't have to wonder why.
If there was any reliable support to be had, I'd love to upgrade my Phantom to a III.