The Schneider Electric A9N21387 is a DT40N miniature circuit breaker (MCB) in the DT40 series, rated 3-pole, 20 A, with a C trip curve. It's a current-production part, actively manufactured.
Snap-mounts onto standard DIN rail inside a panel or enclosure. The 3-pole format handles three-phase circuits up to 20 A per phase. For a panel OEM or wireman, the C-curve characteristic avoids nuisance tripping on transformer or motor-start inrush while still providing short-circuit protection within the MCB's rated breaking capacity.