Weight bracket. The bottom of the weight bracket rest in the hitch hole, then a rod slides through the weight bracket and the square holes on the tractor to keep it in place, then you put the weights on, then a second rod slides through hole in weight bracket only to keep weights from jumping off. Bagger hopper has a different bracket that bolts on near then trans unlock rod.
I'll not be adding weights since this tractor will only be used for mowing or hauling a tow-behind spreader. I'm now curious if I could incorporate those cut-outs when I fabricate a rear bumper. Does anyone have pix showing the nearly-square holes in use?
I'm not familiar with that model, but square holes generally indicates that the holes were designed for use with carriage bolts, which have a square shoulder just below the head, so if you can find the correct size carriage bolts, you should be able to drill round holes in your fabricated rear bumper to line up with the square holes in the frame and use them as mounting points for the bumper. The square hole & carriage bolt combination is good for when you can't easily get a wrench in a tight spot , as the square shoulder on the carriage bolt in the square hole keeps the bolt from turning as you tighten a nut on the bolt.