I haven't yet sawn into it and am still studying where I should do so, but I have taken off some of the enamel and the underlying ivory is very white.

Hippos weren't a protected species in those days, and even in the early days of the CITES legislation, it still wasn't protected. Now it is, though not to the same degree as elephant ivory.
After inquiries to CITES/DEFRA, it was deemed legal to sell it on to me as both the seller and myself are in the UK, but if I do carve it and if ever those carvings are sold abroad, they'll require CITES certificates, which is why it's necessary to know the material's provenance.