The Siemens 3VA9978-0BA21 is a SENTRON-brand shunt trip (auxiliary release) for the 3VA4 and 3VA51 through 3VA64 molded-case breaker family. It trips the breaker remotely when you apply 48 to 60 VAC at 50/60 Hz — no DC variant here, so keep the supply AC. It draws up to 670 VA apparent power on trip, so size your control transformer accordingly.
This shunt trip measures 31.2 mm deep by 21.7 mm wide and 65.1 mm tall — it tucks into the accessory slot on the breaker without stealing DIN-rail space. It occupies three slots inside the breaker accessory bay. Wire stripping length is 10 mm (0.4 in), so trim your ferruled leads to that and no whiskers. The form factor is specific to the 3VA4 and 3VA5/3VA6 frames; it won't drop into a 5SY miniature breaker panel without rewiring, because those use a completely different accessory interface.