FIVE 5-FINGER DRILL BATTERY 21V

tool battery from Mar-Pol. Based on the image, here are the technical specifications and some practical context for what you've got there:
​Technical Specs
​Voltage: 21V (This is the "max" voltage; it typically operates at a nominal 18V).
​Capacity: 2.0 Ah (Amp-hours), which is a standard "compact" size—good for keeping the tool light, though it won't run as long as a 4.0 Ah or 5.0 Ah pack.
​Chemistry: Lithium-Ion (Li-Ion), meaning no "memory effect" and better power-to-weight ratio than older tech.
​Compatibility & Design
​This battery uses the "Power Share" style interface.
​Form Factor: It features the slide-on rail system with a central locking button (the orange tab).
​Cross-Compatibility: While branded as Mar-Pol, this specific physical mold is very common among "generic" or budget-friendly cordless tool brands (often compatible with Makita-style LXT 18V connections, though you should always double-check the pin layout before swapping