SIRIUS 3RA2125-1BD23-0AK6 non-fused motor starter
The Siemens 3RA2125-1BD23-0AK6 is a SIRIUS non-fused motor starter, a direct starter in the S0 load feeder size, rated 750 W at 400 V. It's a current-production part, so you're not hunting for NOS or surplus — this one is still on the factory schedule.
If you're running a high-inertia load that needs a longer acceleration curve, this won't hold; you'd need a Class 20 or 30. The S0 size tells you the physical envelope: 45 mm wide, 155 mm deep, 260 mm tall, so it fits a standard 45 mm slot on a DIN rail or busbar system. It snaps onto 60 mm busbar systems — that's the standard spacing for Siemens 8US busbars. The mounting position is vertical only, so plan your panel layout accordingly.