The bottom of the outdrive housing is sealed with a large, rubber coated metal cup plug - similar to an engine freeze plug if you have ever replaced one. Yours may be like mine and has a plate bolted on over it so you can't actually see the plug while installed on the tractor. I've changed several in my outdrives while I've had them off to replace broken axle shafts and countershafts over the years. An outdrive only takes about 15 minutes to remove but it is a bear to deal with the heavy thing. Your 3510 should be the same as my 4110 although yours is a newer model, it could be different. The plugs aren't very expensive if I remember correctly.