This Monarch 9075 red and clear lens is mounted in the center of the three-light array on the back corners of Airstreams starting in 1975 (some 1974 models). It was used for many, many years in the Airstream and Argosy two-, three-, and four-light arrays.
Also used on Boles trailers starting in mid-60s through to the end of production, 1980s Pace Arrow Motorhomes and 1970s Avions.
It mounts with 4 screws (not included).
Note: The reflector part of this lens can become ineffective over decades of use as the plastic oxidizes. Replacing your lenses is fast and easy and increases safety.