Bed Rail Slide Channel.
This is the bracket that early bed extensions slid into. The original part number is no longer on the package, but we think this was for the plastic slide rail applications.
These bed slides do not use rollers, but rather slide on hard plastic blocks.
Measures 78 inches long by 2-7/8 inches wide. The inside channel measures 1-1/4 inches tall.
Part number 4718-1011