I don't know if the OP is a reloader, but what could be done to determine the chambering is neck up a new case to 30 cal, then run the case up into a 6.5 sizing die that has been backed off, slowly lowering the die a bit at a time and sizing the case until the bolt will just close.  The artificial shoulder you've made should keep the head against the boltface.  Load with a midrange load and fireform to see what you have.  Could be somebody has chambered it to an improved version that doesn't share the same datum line on the shoulder, like Ackley's improved versions do.  In that case, the fireforming operation might be needed to make yourself a supply of safe cases.  6.5x284 is also a possibility.