strange nut from 1945, South Pacific

I was given a seed from the south pacific, somewhere, which my great uncle has had since WWII. I would like to try and identify it with your help.

Dimensions: 4.5 x 5.0 cm (1.75" x 2") near-cirlcular disc, 0.5 cm (< 0.5")thick.

Description: it's slightly less reddish than chesnut brown - maybe closer to mahogany colored. It's split on both sides like a peace sign (like slicing a pie in three nearly equal pieces), but I think that's from applied pressure rather than from a natural split. But, the split is nearly identical on each side, so maybe it is natural. The inside, what I can see of it, is lighter - orangy-tan.

I'm guessing it's a tree seed rather than a smaller plant seed because of it's size. I'm all geussing it's from a hardwood because of it's longevity - it's been 60 years, or so, and it's showing no signs of rot or decay so far as I can tell.

Any info I can search on would be helpful - thanks!