That there is correct; what you had initially posted had a unit error.
You still are ignoring the fact that the bolts holding that bracket on are torqued (to 360 in-lb as per the manual), and consequently, there is preload in the bolts that will create a large shear stress in the threads, just from reacting the axial preload in the bolt, even when no other load is applied. For a lubricated 3/8" bolt torqued to 360 in-lb, there is an axial load of about 4800 lb. The 5100 lb ultimate shear strength of the bolt holes you calculated is probably reasonable, but you must subtract out the preload, so you're down to 300 lb per fastener or 1500 lb total, and that's taking it all the way to failure. Like I said before, we don't want yield either, so using the yield shear stress instead, and a factor of safety, there isn't much of an allowable load left.
This is supported by the CCC service manual for the 1811, 1872, etc. On page 3-28: "The drawbar bracket is designed to pull only light loads such as utility and garden carts not exceeding a steady pull of 75 lbs." Toss a tiller or 3-point bracket on there and you've got the same problems.
You missed my point before...it does not matter what industry this is in because the engineering principles are the same across the board. You can't ignore the fastener preload because this is a tractor and not an airplane. Things get over-engineered because it is not economically feasible to design every component of everything with the minimum allowable margin of safety.
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