These collar-shaped life jackets were worn by SEALs and Underwater Demolition Team (UDT) personnel on maritime missions as far back as the Vietnam War and into the present (sources: http://www.modernforces.com/uniform_UDT_vest.htm, https://www.usmilitariaforum.com/forums/index.php?/topic/364938-vietnam-navy-seal-impression/#comment-2890942). UDT life jackets were intended to be worn around the neck and provide at least 19 pounds of buoyancy (source: https://lifesupportintl.com/products/flo-2304-00) to hold the head above water and can be used to provide emergency ascent during dives (source: https://www.pritzkermilitary.org/explore/museum/past-exhibits/seal-unspoken-sacrifice/modified-udt-life-preserver-pn-110669). The vests are often associated with Visit, Board, Search, and Seizure (VBSS) missions, though the example on this page was found separately from the vest, sling, and pouch on the linked page.
Most UDT life vests have only 1 CO2 inflator nozzle, unlike this one. One and two inflator vests can be seen being used together in the usage photo below.
All metal parts on the vest are non-magnetic, indicating operational use; training examples use magnetic metal parts (source: https://lifesupportintl.com/products/flo-2304-00).
CO2 cartridges are supposed to be attached to the nozzles hidden by the panels. When the cords with the orange balls are pulled, the valve breaks the cartridge seal to rapidly inflate the vest.
Mouth Inflator Stem:
The inflator stem is also provided so the wearer can blow into the vest to inflate it.
Rubber cure date appears to be 6/1989.
This example has been modified with a rubber pad taped to the back straps by the user. The straps are fastened at the bottom with a pair of D-rings on each side.
USMC Force Recon VBSS: