We derive explicit anomaly-index formulas for four-dimensional Weyl fermions charged under the finite symmetries $\text{Spin}\times {\mathbb{Z}}_n$ and $\text{Spin}{\times }_{{\mathbb{Z}}_2^{\mathrm{F}}}{\mathbb{Z}}_{2m}^{\mathrm{F}}$. The strategy is to start from the standard perturbative anomaly indices for $\text{Spin}\times \mathrm{U}(1)$ and $\text{Spin}{\times }_{{\mathbb{Z}}_2^{\mathrm{F}}}\mathrm{U}(1)={\text{Spin}}^c$, and then restrict the continuous U(1) symmetry to a finite cyclic subgroup. On the level of invertible field theories this gives natural homomorphisms, ${\mathrm{TP}}_5(\text{Spin}\times \mathrm{U}(1))⟶{\mathrm{TP}}_5(\text{Spin}\times {\mathbb{Z}}_n),{\mathrm{TP}}_5({\text{Spin}}^c)⟶{\mathrm{TP}}_5(\text{Spin}{\times }_{{\mathbb{Z}}_2^{\mathrm{F}}}{\mathbb{Z}}_{2m}^{\mathrm{F}})$. We compute these maps explicitly by evaluating reduced $\eta$ invariants on geometric representatives of the finite anomaly groups. For $\text{Spin}\times {\mathbb{Z}}_n$, the relevant backgrounds are the five-dimensional lens-space bundle $X(n;1,1)$ and the product $L(n;1)\times \mathrm{K}3$. For $\text{Spin}{\times }_{{\mathbb{Z}}_2^{\mathrm{F}}}{\mathbb{Z}}_{2m}^{\mathrm{F}}$, the relevant backgrounds are $L(m;1,1,1)$ and, depending on the parity of $m$, either $L(m;1)\times \text{Enriques}$ or $L(m;1)\times \mathrm{K}3$. The output is a pair of integer-valued anomaly indices for each finite symmetry. These indices are normalized in the cyclic factors of the finite anomaly group, so they can be used directly in anomaly-cancellation checks for fermions with discrete gauge or global symmetries. They also provide the input for symmetry-extension applications to topologically ordered quantum dark matter: in Standard Model examples, the indices identify the discrete $\mathbf{B}+\mathbf{L}$ mixed gauge-gravitational anomaly left by missing sterile right-handed neutrinos and the finite $K$-gauge topological sector that can match it.

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• Anomalies