This part is a Weaver-like rail to mount red dots on the left side of the RRS plates for the OM-D 1 or the OM-D 5. Mounted like this, you can use both the EVF and a red dot at the same time, using your right or left eye respectively. Depending on your level of training, you might have to close the eye not in use. It mounts using a single 1/4"-20 UNC screw (which you have to obtain separately) on the Really Right Stuff BOEM-1/5 instead of the L-bracket part. Consequently you need the BOEM-1 or BOEM-5 to mount this part. NOTE: The photos show the first version of the part. Meanwhile I added 1mm to it's bottom to provide more support for the mounting hole, moved back the rail slots by 3mm, and removed that last slot completely. All the cheap red dots I've seen so far should still mount on this rail, but please check your red dot mount before ordering this part! V1.2 of this part added a bit of width to the rail, so it fits better when the print comes out relatively small.