Didn't have a name to search on and a number is meaningless to google... but... turns out the "via Guido Rosse 10..." is an address that Google maps can find in Italy and street view shows me Comer Industries. Turns out they make tractor and heavy machinery shafts and gearboxes and other stuff... but nothing like my post hole digger gearbox. I suspect they no longer make that particular product.I would do an online search for that name and number
Granger seems to have seals with about the right dimensions. I am going to use regular grease on the bottom and then fill it with 2 bottles of 00 Grease I picked up today from TSC. Get my holes dug before messing with the seal. Now that I know how this is put together I can pull the seal during the winter and worry about running back and forth to find the right size when I don't need it.
Lesson learned - check the sheer pins first. Just because it turns under no or little load but slips when digging does not mean the gears broke teeth.... it means the sheer bolts have jagged edges and are catching a little. Sadly, slip clutches are expensive so replacing the basic setup is going to wait, though it may be necessary given that I now have a more powerful tractor (RK37HC).