I'll be using a pair of Dominos at each end of the stretcher to join the stretcher to the leg assemblies.  With the stretcher clamped in position, I traced the joint outline onto one of the leg assemblies.  I'll use the long, inside edge of the trace as a reference for a jig.

To ensure that the leg mortises are located in the same position on each leg, I made a fence jig for the Domino.  I grabbed a piece of scrap plywood with a straight edge and clamped it to the leg, aligned with the inside edge of the outline.  Once adjusted as precisely as possible, I traced the edge of the leg assembly onto the underside of the scrap. 

On the drill press, I drilled a series of 5/8" dowel holes, all just skimming the scribed line.   Apparently this piece of scrap had been used previously under something I had been plunge routing.   I think I need to be more selective on what scrap I save.

With some short lengths of dowel installed, I placed it back in place on the initial leg assembly, transferred the location of the top and bottom of the joint to the jig, then used the jig to mark the joint outline on the other leg assembly.  Marking the joint on the second leg wasn't strictly necessary, but it did allow me to take some measurements to verify that both joints were in the same position.

After working out the Domino placement for the joint, I marked their centerlines on the jig, on both sides.

The mortises in the ends of the stretcher are slightly offset from the center since the end is beveled.   I used a 1cm offset from the inside edge in order to match the fixed 1cm distance between the Domino shoe and the center of the cutter.  This is what allowed me to use the inside edge of the joint as a reference line when using the Domino vertically for mortising the leg assembly.

The Domino fence set to the proper bevel angle and set to the proper height to cut 1cm down from the top.

The centerline on the shoe is very faint and difficult to see if the light isn't great.  In preparation for mortising the legs,  I traced the location onto a piece of tape.

With the jig clamped in place, the edge of jig acts as a fence for the shoe.

The scrap of veneer (Yes, I save them, too.  Yes, I have a problem) was used to shim-up the support block to the height of the work.  The same process was repeated on the other leg with the jig flipped over.

Time to check the fit.

Close enough for me.  The bottom doesn't align because I had marked the joint before I trimmed a little off the bottom of the stretcher.

I managed to take this nice sharp picture of the ear muffs with the poorly framed stretcher dry fit in the foreground.

Time to move on to the rear stretcher.