If this ammo is a rimmed round as I suspect it's 6.5x53R Dutch Mannlicher.
It was designed for M1892 & M1893 rifle and was used by the Dutch and by the Romanians.
It has similar power to the 6.5x54 Mannlicher Schoenauer and is a handy little cartridge.
I believe the factory specs are the 160gn RN at 2300fps.
Like the other true 6.5mm/.264" military cartridges 6.5x55, 6.5x54 and 6.5x50R Jap it is a low recoil high accuracy cartridge whose performance on paper belies the fact it's an efficient and highly effective killer.
Dies are made by RCBS and brass can be formed from Remington 303 British brass.
If you purchase a M1892 or M1893 be aware that you have to use clips to utilise the 5 round mag.
Otherwise it becomes a single shot.
On a side note, this rifle cartridge combination was very popular among the gentry in the 1890s and saw use throughout Britiain, Europe and even Africa.
As such companies like Jeffrey and Rigby built rifles using the M1892 action/cartridge for the upper class.