Some folks are beginning to use the designation "OpenCluster" for certain deep space objects called in a dsc file. M21 is one example. It can be found in the Trifid Nebula add-on.
1.4.0 has no keystroke toggle assigned for OpenClusters. Although they are supposed to be treated the same as galaxies, they are not so treated in the build. Toggling a [U] galaxy toggle will not turn them on or off. Toggling an [E] for galaxy labels will not turn off their LABELS. Unfortunately, the default for them appears to have labels for them turned on. There is no way to get the label off, short of renaming them as Galaxies in the dsc file and abandoning their new OpenCluster designation. That would kind of defeat the purpose for creating a cluster designation.
Fortunately, a simple fix. Two new keystokes for OpenClusters need to be assigned. One to toggle them on or off, and another to toggle their labels.
FYI
Frank G