Dell part number 0J120C (or J120C) is a 2.5-inch hot-swap hard drive blank filler/caddy designed for older Dell PowerEdge servers. It is used to fill empty 2.5" drive bays to maintain proper airflow and server cooling. 
 
Compatible Systems:
Dell PowerEdge Servers: 1950, 2950, R900, R905
Other Potential Uses: While primarily listed for the above servers, 0J120C is often used in generic 2.5" SAS/SATA hot-swap bays of that era. 
 
Key Specifications:
Size: 2.5-inch
Type: Hard Drive Blank Filler / Tray
 
Related Part Numbers: J120C, 0HF383 (similar)