The following discussion involves a part that isn’t Colt. However, it is a unique and not commonly known piece of M16/M4 history.
When I began my Army career in 1988, I was a traditional weekend soldier in the National Guard. The hot topic at the time was DSCA…Defense Support to Civilian Authorities.
DSCA was a domestic plan for the National Guard to provide soldiers to Law Enforcement for both natural disaster/non-kinetic environments and potential kinetic environments such as riots.
In a DSCA scenario, Governors and law enforcement officials were concerned with the potential for troops to panic in a violent encounter and rotate their selectors to ‘Auto’ and firing on civilians.
A concept to mitigate the risk of unintended casualties from automatic rifle fire was this selector block that prevented the selector from rotating from ‘Semi’ to ‘Auto’.
In this video, I give you a look at the part, talk about its installation and give you a look at it installed.
The firearm used in the video is the Colt CRM16A1.
Scattered Shots 
FWIW: The selector block was developed and patented by Charles R Fagg of Rock Island Arsenal. Fagg was formerly with the Project Manager Rifles office and continued to work with the M16A1 through the end of his career at RIA.
Fagg later joined with fellow PMR alumni William C Davis to form Tioga Engineering.
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