There are some good demos of this on youtube showing D19/21's using and not using the system and how it aids traction.

   The Ferguson System on the Ford tractor was due to the "handshake agreement" between Henry Ford and Harry Ferguson for the use of Ferguson's system with payment of royalty fees to Harry Ferguson. This agreement carried onto production of the 2N Ford tractor.  When Ford developed the 8N Tractor, Henry Ford decided not to continue paying Harry Ferguson royalties, leading to a long term lawsuit against Henry Ford and the Ford company. Ford eventually lost as he delayed everything possible to drag out things as long as possible. This led to Harry Ferguson nearly going bankrupt, because of extensive lawyer fees and Ford never paid up as ordered by the courts. 

If you can’t do your own depth control you shouldn’t be on a tractor.  

A little more on the late IH FH traction control. 

On the left rear corner of the operators platform is a vertically operated lever with notch settings.It is attached to an eccentric in the FH an that mechanically sets wheel slip. It works. Pull it all the way up you'll reduce wheel slip to zero. And possibly break things.

A contributing factor in the early 560 rearend fiasco. An under speced bullgear bearing( the whole problem in a nut shell), hard working ground and reducing slip to zero through this device. 

   In the 70s we were using M rears in modifieds. We found many things that gave up as we increased the horsepower. First the pilot bearing at the front of the main shaft gave up allowing the top shaft to lift and destroy the transmission gears. So the first fix was replacing the ball bearing with a roller bearing and only giving that bearing a life of about 30 runs. Then we remade the entire input and constant mesh gears with wider gears with heavier bearings also eliminating 4th & 5th gears with a ratio change speeding 3 in an M to faster than 4th in a SuM. Then later with roller bearings replacing the inner axle bearing  and welded in 5th gear we were able to reliably put 1500 HP through an M rear. We did twist axles, but never broke one. 

Since I've owned an early('59) 560 for 34 years I've heard quite a bit. Mine has the update and it's ran the subsoiler two foot deep in clay and hung in there. How IH missed it is a mystery. It was a known problem in 400 and 450's with all the M&W goodies. A liitle known fact is JD had rearend issues with 4010's in severe service but it was handed much more descreatly. This helps explain the amount of iron in the 706/806's.

I think 1956 would be about right. I've seen late 400's with the newer FH. But FH was retrofitable back to the MTA(the rearends were already drilled and tapped. And IH sold them for just thst purpose.

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Ferguson three point was a true improvement to traction and tractor safety ( the reason Ferguson pursued the design, the tractor couldn't flip over). 

+1 Ford-II was the one to break the royalty deal and Ferguson set up a Detroit factory to make Ferguson tractors in the US. Eventually Ferguson merged with Massey Harris.

The AC system is really the Ferguson system with the draft link on the bottom. An upside down Ferguson system that went unnoticed by patent attorneys. That's why it worked.

IH and JD tried other systems. I still remember riding on our Farmall 806 pulling five bottoms when I must have been three years old and we struck a root and that tractor reared into the air. The world was moving the Goliath of a tractor! It had the underslung fast hitch. Obviously imperfect in operation.

When Ferguson patents ran out it became the default for all modern tractors.

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Most of the people who complain about Fast Hitch are like those who complain about Gleaners. They never sat down and read the manual to learn the settings and adjustments.

'52 WD with the upgrade. I've played with the chain. The problem is the hook has very little if any wear. Not a bad problem to have. Hooking up is no problem. Dropping the implement is a different story.

Yes. Doesn't help. I think it's a issue of little to no wear and rust.

I did a little research on this matter. These are the weights of different model tractors from the era discussed in this thread. Numbers from tractor data, heaviest one listed.

Allis)

WD 45 4285 Lbs.

D-17 4670 Lbs.

IH)

M 4858 Lbs,

Super M 5603 Lbs,

Super MD 5770 Lbs.

350 5475 Lbs.

400 6300 Lbs.

450 6519 Lbs.

460 6055 Lbs.

Case)

400 6000 Lbs.

Oliver)

88 5000 Lbs.

Super 88 5400 Lbs.

JD)

60 5800 Lbs.

70 7079 Lbs.

The point of this post is that our AC's are lighter in general than other machines of that time-frame and size. A really good form of traction control was more necessary. I probably pay closer attention to Orange and Red tractors at the plow-days I go to than other colors and what I see is that neither of those have much trouble with traction. Lots of the Red ones are using pull-type plows where traction-control is null. Really don't see many fast-hitch plows for some reason?

Something else, I've heard some knocks on the fast-hitch, not just here even on Red Power that ground clearance could be an issue. The wishbone does decrease clearance compared to a comparable model with three point. Well curiosity hit me. Measured the clearance for the D-17 and the 450, They are just a few feet apart right now. D-17 was 11", 450 was 14". Full disclosure, this is not a fair comparison. 17 has tiny, 13.6x28, rubber on it and the 450 has big, brand new 16.9x38's on it. I think they are about 5" taller than the 15.5's were. I will do some more measuring today for comparisons sake. Will get data from the other D-17 with 16.9's, at least one 45, also with 16.9's, maybe a WD with 14.9's, a 756 with fast-hitch, and an 856 with three point. The last one will be gaining ground clearance next year when the new rubber goes on.

As to why people don't use FH plows. Except for the coulters nearly all the adjustment is in the FH itself. Once you READ THE MANUAL it's pretty straight foward. I am by no means a plowman but I know what it's supposed to look like when you're done and both my 560 and 706(former) do a great job running a consistent 7+ inches deep. The 560 especially. Hock it in as deep as it will go,set the traction control and go. Pull the T/A occasionally.

Everything I measured yesterday was between 13" to 15". This kind of surprised me. Of course if these were taken on concrete the measurements would vary a bit. Really thought the 856 would have more clearance than the others, it has a little more but not much. It will gain a couple inches next year when the new rubber goes on however!

The fast hitch does look like it could be a pain while raking/baling hay.

It really isn't. I use a #16 IH rake and my buddy uses a #35 behind his 560. I have 16.9's he has 18.4's. Just don't lock the FH and you can raise and lower to the perfect rake height.