We consider a three-parameter family of linear special double confluent Heun equations introduced and studied by V.M.Buchstaber and S.I.Tertychnyi, which is an equivalent presentation of a model of Josephson junction in superconductivity. Buchstaber and Tertychnyi have shown that the set of those complex parameters for which the Heun equation has a polynomial solution is a union of the so-called spectral curves: explicit algebraic curves in C 2 indexed by ℓ ∈ N. In his paper with I.V.Netay, the author have shown that each spectral curve is irreducible in Heun equation parameters (consists of two irreducible components in parameters of Josephson junction model). Netay discovered numerically and conjectured a genus formula for spectral curves. He reduced it to the conjecture stating that each of them is regular in C 2 with a coordinate axis deleted. Here we prove Netay's regularity and genus conjectures. For the proof we study a four-parameter family of linear systems on the Riemann sphere extending a family of linear systems equivalent to the Heun equations. They yield an equivalent presentation of the extension of model of Josephson junction introduced by the author in his paper with Yu.P.Bibilo. We describe the so-called determinantal surfaces, which consist of linear systems with polynomial solutions, as explicit affine algebraic hypersurfaces in C 3 indexed by ℓ ∈ N. The spectral curves are their intersections with the hyperplane corresponding to the initial model. We prove that each determinantal surface is regular outside appropriate hyperplane and consists of two rational irreducible components. The proofs use Stokes phenomena theory, holomorphic vector bundle technique, the fact that each determinantal surface is foliated by isomonodromic families of linear systems (this foliation is governed by Painlevé 3 equation) and transversality of the latter foliation to the initial model.