The Siemens 5SL4263-8 is a 2-pole miniature circuit breaker from the SENTRON 5SL series, rated 63 A at 400 V AC with a D tripping characteristic.
The D-curve trips magnetically between 10× and 20× In (630 A to 1260 A), so it's a deliberate choice for circuits with starting surges — a motor starting direct-on-line or a bank of discharge lamps. At 30 °C the breaker carries the full 63 A; at 40 °C it derates to 59.67 A, and at 55 °C to 54.29 A — you'll need to account for enclosure heat rise if the panel runs warm.
Integration notes
Sealable with a lead or tag to prevent unauthorized switching.