When ever I see 7mm I first think of the 7mm Mauser or 7X57 . There are a couple other European 7X (various). Lots of European cartridges are named like this. The first number is the diameter or caliber of the bullet in milimeters (mm) and the second number is the lenght of the cartridge case in mm' s.

A 7X57 is about like a 7mm08.
A 7X61 Sharp&Hart is about between the 280 Rem and the 7mm rem. mag.

Robin