Because somebody is going to ask how old that "R-P" marked cartridge-casing is... the Peters Cartridge Co. was purchased by Remingtom Arms Co. in 1934, which became Remington-Peters. That company is still marking casings with an "R-P" headstamp today.
8mm Mauser had a sudden boom (no pun intended) in popularity in the early 90's when tens of thousands of captured German K98 Mauser rifles were imported from warehouses in Russia and other former Soviet countries. Your cartridge doesn't look very tarnished so I'd guess it's not very old at all.