Ian McCollum of Forgotten Weapons posted an interview with Larry Vickers. They discussed his Colt 723 carbine that he was issued when he was first with SOF-D aka Delta Force.
Ian says in the description for this episode:
Use of carbines like this one by Delta and other special forces groups set the stage for the adoption of the M4 Carbine and Aimpoint M68 optic by the US military at large, and it’s very interesting to listen to Larry’s first-hand experience of how and why it was put together.
By the way, if you want to duplex magazines like that, Matt Bracken (Enemies Foreign and Domestic) has an excellent “how-to” article on it here. I’ve done it with black duct tape and a thin dowel.