Well, it’s been a few weeks, but I have drained the engine, flywheel/clutch, and gearbox. Opened up the gearbox, which I found to be amazingly clean. My father said that the person he bought it from had re-built the engine, so they must have also opened up the gearbox and cleaned it. My father did not recall doing it. After getting everything drained and checked for sludge, I drilled and tapped a 1/2" plug at the location Joe P. indicated above. Then filled the gearbox/flywheel and clutch with 4 gallons and 1 quart of the same oil as I am using in the engine (15w40 diesel). Interestingly, the oil level with that amount of oil is about 7/8"-1" below the plug. Still a good reference/overflow point. Looking in from the "back access" by the right brake, the oil level is just over the second tooth from the top of the belt PTO when turned counterclockwise to take up the backlash. Anyway, it's back together with proper fluids and fluid levels. Kind of a bizarre learning lesson, but now I know..........