One marked-sum law determines random mass partitions and $\mathrm{\Xi }$-coalescents

Let $P=(P_j)$ be a random mass partition and let $(X_j)$ be iid real marks, independent of $P$. We construct a fixed mark distribution for which the law of the real random variable ${\sum }_j{P}_j{X}_j$ determines the law of $P$ on the Kingman simplex. The observation map is an affine topological embedding. One construction uses an infinite convolution of stable laws with rationally independent indices; for every prescribed $q\in (0,\mathrm{\infty })$, a determining mark can instead be chosen symmetric, centered, compound Poisson, and in $L^q$. At any known positive time, the law of the ranked block frequencies of a $\mathrm{\Xi }$-coalescent started from singletons$\text{unicode}x2014$equivalently, the exchangeable partition probability functions of all its finite restrictions at that time$\text{unicode}x2014$determines the finite collision measure $\mathrm{\Xi }$. Composing the two results therefore identifies $\mathrm{\Xi }$ from the law of one real marked sum. Within the $\mathrm{\Lambda }$ subclass, an asymmetric Bernoulli coloring determines the normalized collision measure, as does any centered nonconstant mark whose absolute moment of some order $a\in (1,4)$ is finite while every higher absolute moment is infinite. We also prove that, for every $d\ge 2$, a pair of distinct laws supported on strictly decreasing $(d+1)$-tuples of positive masses with a fixed total mass remains indistinguishable for every mark supported on at most $d$ points. For every fixed number $d\ge 2$ of types, this obstruction yields distinct normalized collision measures with the same mutation-free neutral $d$-type $\mathrm{\Xi }$-Fleming$\text{unicode}x2013$Viot transition semigroup.