It does not fit into a receiver and does not require "O" ring. That is a standard brass ell that is secured into PEX tubing with a stainless steel clinch band. The tubing in the picture is the ell being held on with a clinch band.
Hallmark used that type of material in the water lines including the shut-off valves for the water line drains. Do-it-yourselfers now usually use Shark Bites to replace them because they are simpler to use and do not require bands. When using clinch bands the shift now is to the black plastic ones you find at Lowes and Home Depot.
If the OP can locate the line it came out of, assuming the band holding it let go, the easiest thing to do is cut off the clinch band showing in the picture and replace the ell with a Shark Bite ell by simply sliding it onto the tubing showing in the picture and the piece of tubing the ell came out of. Should only take a few minutes. Easier way to cut off the band if you do not have the clinch tool to twist it off is using a Dremel with a small grinding disk, but you can also use a hack saw. Just cut it half way and then twist and it will break off.
