If the gun is not marked "70" on the barrels it was originally chambered for 65mm (2-1/2" or 2-9/16") shells.
The topic of longer shells in shorter chambers has been extensively discussed here on the board as well as being covered by several articles in Double Gun Journal. The gist of all the testing was that chamber length by itself really doesn't affect pressures that much, at least with modern plastic shells and wads, but it also demonstrated the important point that nearly all modern factory 2-3/4" ammo is loaded to pressures in excess of what you want to be feeding older guns no matter what their gauge or chamber length.
Find yourself some verifiably low pressure shells and enjoy your new old Greifelt!
