Top: quantity R_κ(θ) (Eq. (33)) calculated for an NIS with external shear γ_p (cf. Sect. 4; solid black) and for various SPT-transformed models with SPT of the form 1 + f₂/ 2 (β/θ_E)² (Eq. (28)). The range of positive values of f₂ allowed by | Δ α_max | < 5 × 10^-3θ_E (Fig. 4) is explored for two different choices of the shear: γ_p = 0.05 (blue) and γ_p = 0.1 (red). While R_κ is conserved under an MST, it is not under an SPT, with deviation that can reach tens of percents. Bottom: for each curve of the top panel, we show the difference between R_κ of the original NIS model and of the SPT transformed model.