  Consider a $q$-Weil polynomial $f$ of degree $2g$. Using an equidistribution
assumption that is too strong to be true, we define and compute a product of
local relative densities of matrices in $\rm{GSp}_{2g}(\mathbb{F}_\ell)$ with
characteristic polynomial $f\mod\ell$ when $g$ is an odd prime. This infinite
product is closely related to a ratio of class numbers. When $g=3$ we
conjecture that the product gives the size of an isogeny class of principally
polarized abelian threefolds.
