It just allows more room to grasp the bolt handle. World wide, most Mausers probably had straight bolts, and not all those with bent bolts had the stock cut out.
A bolt modified for a sporting rifle will have to have a different type of bent bolt or welded on bolt handle to clear a scope sight; the curved K.98k bolt handle won't clear a low mounted scope.
I don't know of a Mauser stock identification guide per se; Ball's Mauser Military Rifles of the World is a good book on Mauser bolt rifles in general.
Jim
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