It seems to me, my dearest sir, you’ve stumbled on a mystery.
To figure out what this is for, a brief lesson in history.
I’d say it was just a few years hence.
When Flotilla specs were on the fence.
And Pibrella it was on the brink.
Of hitting shops, we had a think.
And tossed upon its boarded edge.
The connector choice t’was in our heads.
But it turned out Flotilla was delayed.
As perfections, changes and decisions made.
So never did it find a use.
And now you’ve found an end that’s loose.
This is a pretty poor excuse.
For a poem, it’s quite obtuse.
But if you’ll read just one more verse.
I’ll give you a TLDR, far more terse.
The 3V3 comes from the Pi. It doesn’t really matter where, but I could look it up if you’re curious. The connector is, in fact, a very early choice of Flotilla connector, with the idea that a Flotilla module could be connected to the Pibrella to extend it with extra sensors. So that’s why it’s an unconventional pinout and landing choice.
Unfortunately we updated Flotilla to use 5V for technical reasons, so you wont be able to hack Flotilla modules onto it, but you can use it for I2C devices that run at 3v3. The Pi has onboard pull-ups, so it should be simple enough.
I think it’s a 0.1inch pitch landing, so any 4-pin connector with 0.1inch pitch should work. I’m not sure where the best place for you to pick one up might be, though!
Alternatively, you could just solder wires right on, maybe the female end of a group of 4 female->male jumper jerky wires.
Okay… even the TLDR was long!