Tried to search turtle shells by species. But due to the patination on the 'turtle shield' it is very difficult to pin point one in particular. I can tell you the likely suspects for providing the shell have nesting sites on the African coastline bordering the Red Sea.(Egypt, Sudan and Ethiopia) Unfortunately, the same species are found around Australasia, Polynesia etc.
So no conclusive answer, I'm afraid.
Actually wondering whether the shell would be a good shield........it didn't protect the previous owner very well...