Could be either/or depending on when it was made. If you post the serial number (i.e. F6847XX) we can tell you when it was made and possibly determine if it is indeed large or small shank. However, given the parts finder tells you it's a large shank, I'd tend to think Savage knew what it was when it left the factory.

Savage first started chambering the .300 Remington Ultra-Mag's (RUM) in 2001 and offered it through 2005. In 2002 they added the RSUM cartridges to their offerings, but they were dropped at the end of the same year. The Winchester Short Magnum (WSM) cartridges were introduced in Savage rifles in 2003, and the first ones were built on a small shank. As such it's safe to presume that the large shank didn't materialize until sometime in mid 2003 or later.