The 1.5KE20A R0G: That clamp voltage is what the downstream IC's absolute-max rating must survive — if the IC is rated 30 V, this TVS buys margin; if 25 V, the clamp is too high and a lower-voltage part is needed.
DO-201AA (DO-27) axial-lead through-hole package, supplier device package DO-201. The 0.042-inch (1.07 mm) lead diameter fits standard 1.2 mm PCB holes; the body is 9.1 mm long and 5.3 mm diameter. Bend the leads 90° for horizontal mounting or leave straight for vertical — the axial form factor is common on power-entry boards where the TVS sits after the input fuse and before the bulk capacitor.