We have two different types so far.
This one was found on a three-neck lap steel (see that here):
This one was discovered on eBay. It attaches to an acoustic guitar. The two black knobs are Volume and Tone. I think the left side (in this picture) abuts the bridge of the guitar, and the right side goes against the neck side of the sound hole. The pickup can be slid back and forth for ideal positioning.
The tuner at bottom is is unrelated to the pickup: