Jim, get out your ruler/tape and measure the holes (size and distance between) where the gearbox mounts to the deck. Go to a nearby TSC and measure the same dimensions on one of their gearboxes. I know Howse and King Kutter use 40 hp gearboxes that are interchangeable because I swapped one from a Howse to a King Kutter.
I say all this because if you cannot ID the cutter, even finding the brand might not be much help. There may be no parts available. This is the best advice I can give without a picture of the cutter showing specific details like if it has a stump jumper and how many teeth are on the output shaft going inside the stump jumper. I have an old mower with a good gearbox and mangled blades. The spline is 11 teeth, a bit of an oddball. You just never know. When an old mower like this breaks, it's sometimes better to haul it to the scrapyard and go buy a new one or a used one of known brand with parts support.