what you have is indeed an M 88 sporting rifle

up to WW 1 the common designation for the 8x57I cartridge was "Mod. 88 8mm"

because it was the cartridge for the M88

the 7x57 Mauser was first chambered in the spanish Model 93 Mauser so the designation was Mod. 93 7mm

see it standing here under the cartridge illustration

so your rifle is chambered for THE Mod. 93. 7mm cartridge in a DWM made M 88 action

there is a thread for pics of M 88 sporters, please add some more of your rifle.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=188673&page=0&fpart=2&vc=1
the M 88 in 7x57 is rare so I suspect it was made for export to south africa because the cartridge was very popular there. is it still for the M 88 clip?
I remember another M 88 sporter that was made for export to argentina and chambered for the 7,65x53Mauser.