Well ... turns out it's pretty easy to reach someone at SME. I called and they told me that -- as you suspected -- this is not the balance weight that came with the arm. They sell five/six weights and this is one of the heavier ones. It's authentic -- just not the one that would have come with the fixed-head shell version of the arm.
I can order a lighter one -- for $200 and a four- to six-week wait. So that's lovely.
He said not to just add weight to the headshell and use it that way.