I can't really tell if the ends of the stretchers on the original chair had shoulders around the tenons or not.  I decided that I would use shoulders to simplify clamping later in the project.  Because the three stretchers meet the tapered legs at different heights, the distance between left and right shoulders will be different for each stretcher.  They'll have to be cut with precision to ensure that legs will be plumb and square after glue-up.

I'm also planning for integral tenons on the stretcher ends for strength because there will be mortises about an inch inboard of the shoulders to accommodate the two front-to-rear seat frame stretchers.  Using a floating tenon on the ends of the stretchers would really compromise their strength since there would likely be intersecting mortises.

I used my cutting mat to lay out the positions and outlines of a pair of left and right legs.  From the pencil marks on the mat, I could place a stretcher blank at the design height and transfer the marks for the shoulders.  I'm using a scrap 2x4 sized properly to validate the process.   With the shoulder locations marked, I used a batten to lay out the upper and lower curve of the front stretcher.  When the excess is eventually cut away and the curves sanded smooth, I can use this piece as a template for the actual stretchers.

In preparation for cutting the tenon, I cut off some excess to make the router's job easier.

I setup the Leigh m/t jig with the vertical fence at 3.4 degrees off-plumb to make the shoulder line horizontal.

With the end of the 1/2" spiral bit aligned to the shoulder line, I cut the tenon.

The tenon is shifted slightly to the front side in order to leave the stretcher with a small reveal when inserted into a centered mortise in the leg.  On the back side at the end of the stretcher, some material will be removed to yield a similar reveal at the leg.   This seems to be what was done on the original.

Something like this:

As long as I'm careful about aligning the router bit to the shoulder line, this should give me the precision I need.

I almost went straight to cutting the mortise in the leg because that is pretty simple, but good sense won out and I tried a scrap first.   

I'm glad I did because the combination of the 4" long spiral bit and the deflection of the Porter Cable 6391 plunge base gave me a mortise with two much slop.  Apparently, that's a big complaint about that base.  I got the router in the bundle with the Leigh jigs, so I've not had a lot of time using it.  I'm now paused until I can get a different plunge router mounted to the jig plate.   I'm going to see if I can add some mounting holes to the jig plate to accommodate my Festool router.  Once that's done, I'll get back to the stretchers.