Symplectic leaves in projective spaces of bundle extensions

Fix a stable degree-n rank-k bundle F on a complex elliptic curve for (coprime) 1≤k<n≥3. We identify the symplectic leaves of the Poisson structure introduced independently by Polishchuk and Feigin-Odesskii on Pⁿ⁻¹≅PExt¹(F,O) as precisely the loci classifying extensions 0→O→E→F→0 with E fitting into a fixed isomorphism class, verifying a claim of Feigin-Odesskii. We also classify the bundles E which do fit into such extensions in geometric/combinatorial terms, involving their Harder-Narasimhan polygons introduced by Shatz.