"When did R/P begin making ammo for this British round?"
PaulS,
To answer one of your questions, this is a .303 Savage not a .303 British. There is a considerable difference.
UPDATE EDIT: Rem-UMC headstamp was replaced in 1960 to the R-P headstamp which is still used today.
Looks like the old relic wasn't as old as first thought. Thanks for pointing that out Paul!
Bill
ps. I don't have a Savage 99 in .303 SAV so I won't be trying to fire the old round...(wouldn't anyway).