I'm hoping that it was something left in the engine by mistake and was kicked around by the crank shaft, but I have not rebuild a briggs and stratton in a long time, don't know what's changed inside. I'm told this is a replacement engine for the those mower, so it's second engine.
Used to crank hard, burned up the starter, new starter works great though, was a free mower, and I'm sure it was run with little or no oil because the valve cover leaked bad onto the muffler after i filled the oil, Fixed that and noticed under the fuel tank was now more covered in oil, I figured...
This came though the back of my 19hp craftsman briggs and stratton engine, there is a hole, I'll fix that, but what is it? It's half round, flat on one side, looks important but the engine still runs, other than blowing oil out all over the place.