FN52, thank you for the excellent photographs.
From your first post I gather you want to know a little more about .30-06. Originally it was introduced in 1906 in the U.S. and the variation you show ist the ball M2, which was the standard cartridge in WW2 and the Korean war. An armour piercing version with a hardened steel core, identified by a black bullet tip and called AP M2, was also in wide use by U.S. troops. Even today the caliber .30 AP M2 is used as a standard in penetration tests.
Many countries used U.S. surplus weapons and ammunition after WW2. Most probably this FN 52 was made for one of them. Sorry, it is by no means rare.
The box it came in is not intended for small arms ammunition, but for an item designated M19A1 (a fuze, a hand grenade, or whatever). Others on this forum will know what it is.