Compaq 3U RM Storage Enclosure Reference Manual page 23

4. From the rear of the rack, insert the front tabs of the spring-loaded rack
rail into the proper holes.
5. Compress the rail forward and insert the rear tabs of the spring-loaded
rack rail into the proper holes. The rail locks into place securely.
Figure 3-2. Aligning and inserting the spring-loaded rack rail into the front
holes and the rear holes (rear view)
6. Repeat steps 4 and 5 for the second spring-loaded rack rail.
7. From the rear of the rack, align the cable reel with the hole in the right
spring-loaded rack rail and secure it with an M6 x 1.0-12L Phillips
screw.
Figure 3-3. Attaching the cable management reel
8. Tighten the thumbscrew into the spring-loaded rack rail.
Installing the 3U RM Storage Enclosure into a Compaq Rack 3-3