How did you manage to purchase a rifle in .300 WSM in 1991 when the round didn't exist commercially until 2001?

As for the bolt handle fitment, the design hasn't changed since 1966. All the companies out there who supply parts for Savage's will basically give you a cut-off date (usually 1996) as that's when Ron Coburn took ownership of Savage out of bankruptcy protection so they aren't liable/responsible for anything made prior to that. Also, given all the small changes Savage has made over the years such supply companies have no way of knowing what will/won't fit a given rifle of a given vintage. Last but not least, most such suppliers only have access to current production parts as that's all that's available to them from Savage. So for example, if you have an older rifle with the stagger feed magazine system and you need a new magazine follower, you aren't likely going to find one for sale from any parts supplier (Numrick's being the exception since they disassemble and part out old guns).