Actually Jim, it's an anthocarp, a mauka anthocarp. On one of my mauka roja plants. I'll put the tricorder down for a minute and explain. An anthocarp is a specialised fruit found in the family Nyctaginaceae to which mauka (Mirabilis expansa) belongs. And before you ask, yes, I do know what a nail brush is.
Seems like my clumsy fumblings have resulted in a single, successful pollination. Alternatively, it may have happened spontaneously, without any connection to the prodding and poking which I doled out to all the hapless flowers I encountered.
What looked like pink petals in my previous picture weren't - they were colourful sepals standing in for the petals, which went AWOL sometime in the evolutionary history of this family. Similarly, what looks like a ripening seed is, from the botanist/pedant's viewpoint, a fruit, albeit somewhat unlike the common conception of one. It's an achene (dry fruit) wrapped in a persistent calyx - known in botanical shorthand as an anthocarp. In mauka's case this outer wrapping is covered in sticky hairs. These must help the ripe seeds stick to any passing birds or animals which then distribute them to new areas.
Here's a close-up of the achene, divested of its glandular, sticky coat. There are definite ridges present on its surface - a common feature in Mirabilis species.
So, although I don't know whether this mauka propagule is viable, it seems likely that, under some circumstances at least, mauka can self pollinate. This can only be a good thing.