The paper investigates the decomposition of the pencil of skew- symmetrical forms \[ (\phi,\psi)\mapsto \int \phi (x)(-d^ 3/dx^ 3+4(u(x)+\lambda)d/dx+2u'(x))\psi (x)dx \] in the space of functions on \(S_ 1\) into irreducible components. In the case of odd or infinite dimension the Kronecker component is defined and the corresponding pencil of operators is constructed. Further, the models of this pencil in the spaces of sequences and spaces of sequences of entire functions are constructed. The paper studies the questions concerning the isomorphism of the model operators. In particular, it solves the question concerning the isomorphism of forms.