Coupling added

I finsihed the 5/16 coupling and added grooves to the top and bottom of the vent plate.

The 5/16 text as seen on the first screenshot is as close as I could get the original part, the font may be slightly off but not by much, however this is only used on the original props for the right side ear piece. The left one has what looks like an "8" on it. 
As they probably cut a real coupling in half and glued it in place it makes sense that the writing would be different on each side. I

I can also see that the top of the coupling is too wide. I'll change this.