Its actually quite common, esp on the 10" augers, but its happened on the 8" as well. The drive shaft is actually pipe from the gearbox to a few inches from the top where a stub shaft is welded on. The welds never break, its the pipe right below where the stub shaft is will twist off. The pipe is a bit light considering the torque on it in a 10" auger, but Meridian wasn't too keen on taking any advice from customers.
Remove the top cover, chain, sprocket and the plastic red cover will slide off. Its not a terrible fix, but you've got 3 options.
1.You can either install a heavier wall pipe and lathe down the stub shaft to fit.
2. Use a collar with a shear bolt/pin.
3. Weld on a collar.
Considering the model number you provided and its age, it went quite a while until it broke. Your probably fine with a collar. The one auger I have with shear bolts seems to always break them at the most inconvenient times no matter the quality, so I'd stay away from shear bolts.
Never seen an auger with a shear bolt except on a swing away