The shaft didn't break from this one incident. The lines you see on the fracture surface are called beach marks. They are created as the shaft fatigued. The rough area in the center is the final fracture. There is a step on the shaft OD where the shaft was machined - probably the snap ring groove. If you look at the JD Parts Catalog, it shows 3 possible gear boxes. The ones shown as item 28 and 30 illustrate a shaft with a snap ring groove. Item 32 is the later design (shown by the serial number range having no end number) and the snap ring grove has been removed. The illustration shows that instead there is a shoulder in the middle of the shaft where the snap ring was previously located. It is very likely that they found the original design was subject to bending fatigue failures. That snap ring groove not only weakens the shaft by the reduction in diameter, but the corner of the groove creates a stress riser that magnifies the fatigue factor. The new design is much more expensive because it is made by machining the shaft out of a larger diameter bar, but my rough estimate is its at least twice as strong as the original design in the loading that a rotary cutter sees. It looks like you can get a replacement for the failed part if the parts catalog is correct - it doesn't say anything about not being serviced separately - that you have to buy the entire box, blade holder, and blades, but it looks like something on which you would expect a repeat failure. A shear bolt or lower slip clutch setting isn't going to stop a fatigue failure like this - it is going to happen sometime although a lower slip clutch setting will prolong life.