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.
The Colt AR-15 Resource 
FWIW: It was patented by a RIA employee named Charles R. Fagg during the mid-1970s. Fagg had previously worked in the Project Manager – M16 Rifle office before its closure. After retirement, Fagg worked with William C. Davis, Jr. at Tioga Engineering.
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Thank you for stopping by Daniel and offering additional insight.
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We had them on our Colt SMG 9’s, except ours were made of sheet metal, during qualifications, the armorers and a couple of us would stay after to clean the weapons. Somehow every time someone would bend it back and forth til it snapped and could be put into auto! It was a fun gun to shoot in full auto
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Cool
I can think of 2 times I could have used one of those. 1st as a guide to drill the selector detent hole in my Nodak Spud 80% lower. 2nd to drill the same hole in a couple of Lonestar Ordanance Stowaway paintball grips I came across.
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