I think that the row of finger joints between the arm and leg is a perfect example of honest design.  The joint provides incredible strength to the arm/leg assembly, and Wegner did not try to conceal it.  In fact, I think he celebrated it, given the width of the joint.  It's a very distinctive and necessary part of the design, and I'll try to replicate it the best I can.

I have a second-hand Leigh D4R jig, and I've not used it for finger joints yet, so this will be a first.  I'll use a finger width of 1/4", which seems very close to the spacing of the original and allows me to duplicate the total of 13 fingers across the joint.

I found out that the Leigh jig was not intended to work the way I needed it to for this particular project.  Fortunately, I was able to work around its limitation.   My issues were:

With at least having worked out how to cut the fingers in the legs, I first started setting-up the router and special guide bushing to get the fingers fitting together snugly.  I used some scrap 2x4s sized to the target width and thickness, followed the great Leigh setup instructions, and the first attempt was right on the money.

With that done, I made the small wedge mentioned above for the work to rest against.  The thin end cut off of the edge I inverted and taped to the clamping bar.

I glued a 1/8" plywood spacer to the side of the wedge.  This would then register against the tiny fixed fence to shift the work to the right the required amount.

Then a trial run on the scrap wood.  After a little trial and error, I was getting the results I wanted.   

I'm using red oak for the chairs, from the same stock as I used for the media cabinet project.  It's got a bunch of old borer holes.  I have to embrace them and say they'll add character.

I planed all the leg blanks to 1" thick and cut them to 3.5" wide.  I cut the 7.8 degree bevel at the top end and left them long in case there were any catastrophic errors when routing the fingers.  I could cut off the error and start again.

After loading-up the first oak blank in the jig, I found that it was slightly off-center, so I tweaked the position of the comb a little, and then made the first cuts.   

While the jig has a dedicated 1/4" spacing comb, it's too shallow to cut the 1" material.  Fortunately, you can use the 1/2" comb using a 1/4" bit, cutting a first pass, then sliding the comb down to position for the second pass.   An index pin through the comb into the stationary spine sets the spacing exactly.  Here is the result after the first pass. 

When I went to slide the comb to position #2, I found that my last-minute fiddling to center the comb over the work left too little room for the comb to slide fully into position.  The right side of the comb was hitting the stop at the end before the position #2 hole could move fully into place. 

After kicking myself, I thought about it for a while, and decided I could remove the comb without disturbing the settings, and file down that right edge to give it a little more room to slide.  It was kind of a pain, but it worked.

Crisis averted, I completed the final cuts.

With the fingers cut for the first leg, I repeated the process for the remaining legs.  This time with no problems.  Because of the tiny fence, it was easy to be off-plumb when clamping-in a new leg blank.  I had to check for square against the bottom of the comb and adjust for each leg before starting the cut.  I'll need to work out a better way when cutting the arms.

The diligence paid off.

Checking fit with a pair of legs.  So far, so good.