Here is an interesting Colt AR variant that I came across on Gun Broker this morning. It is indicated in the auction that this is number 18 of 100.
https://www.gunbroker.com/Item/881055473
The Colt AR-15 Resource
"A site dedicated to the history of the Colt AR-15 rifles, carbines and associated items"
The Colt AR-15 Resource 