Ypsylon, thanks for your response. I noticed that there are allen bolts behind the rear axle. Was wondering if when tightened they pulled the rear wheel back to tension the chain. Some motocross bikes used such mechanisms in the past.
It’s a BMX chain tensioner that we modified a bit. You’re right, it’s a washer with a bolt that tightens against a plate that is braced on the back of the horizontal drop out. Very similar to what some motorcycles use. They come in a few different types and shapes but they’re all pretty much the same concept.