Pictures (Barthel):

This tablet and Large Washington Tablet (S, RR 16) were bought in 1886 'after a great deal of trouble and at considerable expense' from Alexander Salmon on Rapa Nui by the American Paymaster Thomson. (Fischer)

The type of wood is not known. Fischer indicates that perhaps both sides once had 9 rows. I agree, because 8+9 = 17 is an odd (unlucky) number.