8x57mm is 8mm Mauser (7.92x57mm to be specific but also aka 8x57)
Going by memory I think the conversion is 15 grains per gram, so 15.5 grams bullet weight comes out to about 232 grains. Checking my reloading manual I see data on Hogdon.com for up to 220 grains, so you likely have the heaviest bullet available for 8mm Mauser.
If you're worried or suspicious and need to be further reassured, pull the bullet out of the cartridge and mic it, it should be about 0.323" in diameter. You could also weigh it if you have a reloading scale and make sure the bullet weighs 220 or 230 grains. I'd say you're ok though without doing all that, since the package labeling is pretty unambiguous to me.