Manhattan Bridge BMT turning loop?

On the south side of the Manhattan Bridge, north of the former Myrtle Avenue station, are apparently remains of a planned turning loop. These remains are shown as stub tunnels on vanshnookenraggen’s track map.

However, on nycsubway.org, the description for DeKalb Avenue station (on the BMT 4th Ave Line page) implies that the loop tunnel was fully built, but tracks were likely never installed.

Where else can I find information about this loop? I’m curious and want to know more, like if it was actually constructed or not. If the tunnel does exist, and actually does loop all the way around, are there photos?