Hello there! I'd like to show you a picture:
Yes, you are correct, you have seen one of these before. Hmm. Almost correct: this one is slightly different:
What on earth?? Is that... a strap-pin jack?? Yes, yes it is. That's a 1/4" hole that accepts a standard 1/4" audio plug, such as might be used to connect to an amplifier. Inside there it's attached to a piezoelectric pickup, adhered to the soundboard. You can't see that in the following image either:
We've had this all planned out for a long time, but it's taken a while to get all the pieces to come together. I cannot express how awesome it is to play around on one of our little ukes, enjoying myself as usual, and then plug it into my amp and enjoy myself louder... and with reverb... and other awesome effects! Super fun, unbelievable fun.