You found it.
The dipstick is for the hydraulic fluid, and the rubber plug is for adding fluid as needed. Use MF's Permatran for the fluid. It's known around here as liquid gold. There are probably other fluids that are rated the same, but for my money I would not chance it.
If you want to actually change the fluid, lower the three point hitch first, and then drain the oil by removing the drawbar (if you have one) and removing the drain plug directly underneath the drawbar. It helps to have the tractor on a slight up hill incline; I actually jacked up the front end of mine when I did it to get it to drain more thoroughly.
Capacity is 4.5 gallons per my eBay original owner's manual. It doesn't reference a hydro drive model at all, but mine is gear drive anyway. I don't know if a hydro takes more fluid or not. You won't get all of it out the drain though, so you probably won't need that much to refill. If you are driving other hydraulics with the fluid, you probably need somewhat more fluid to refill it.
The hydraulic fluid filter should be visible inside the left rear wheel fender, and I use a Wix 51210 or NAPA FIL1210. I was shocked to find my NAPA dealer had a listing on this tractor, so I went ahead and bought all my filters from them. Of course you can use an MF filter too. If you have a hydro check with the dealer. There may be additional filters or different ones for those models.
Way more information than you probably wanted, but thought I'd share what I know from working on my own 1010.
Have fun working on it. Keep us posted!