Given a bounded smooth domain Ω ⊂ R3, we explore the relation between couplings of the free Dirac operator −iα · ∇ + mβ with pure electrostatic shell potentials λδ∂Ω (λ ∈ R) and some perturbations of those potentials given by the normal vector field N on the shell ∂Ω, namely, {λe + λn(α · N)}δ∂Ω (λe, λn ∈ R). Under the appropriate change of parameters, the couplings with perturbed and unperturbed electrostatic shell potentials yield unitarily equivalent self-adjoint operators. The proof relies on the construction of an explicit family of unitary operators that is well adapted to the study of shell interactions and fits within the framework of gauge theory. A generalization of such unitary operators also allows us to deal with the self-adjointness of couplings of −iα · ∇ + mβ with some shell potentials of magnetic type, namely, λ(α · N)δ∂Ω with λ ∈ C1(∂Ω).

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