Seed-pod fun

While out hiking in the San Gabriel foothills last week, I came across this seed pod. It grows on a creeping, morning glory-like vine and, when discovered, it was fully sealed.



After keeping it at home in a dish for three days, the bottom split open (as suggested in the image) to reveal four massive brazil-nut-like seeds. When the seed pod dries (more images to follow!), it resembles a luffa.



In fact, looking online, I find this image of a species of wild luffa native to subtropical regions like Vietnam, which certainly resembles our specimen. But, how have they preserved the skin of this specimen so that it doesn't rupture? Possibly, this specimen was picked before it grew the thorns so perceptible in my object? In any case, if anyone can help with the identification of this curious beast, Old Ken would be most grateful.