While taking a break from my other ongoing disasters.....clutch fluid, master cylinder, fuel pump, etc......, I thought I would just do something simple and change the trans fluid in my 2200 5-speed manual. Crawled under there, found the drain and level check plugs (or what I *thought* were those plugs), and cleaned off the fill plug at the top next to the red vent cap. Grabbed a 5/16 allen key and it fit tightly and snug in the top (fill) plug but I couldn't get it to move at all. So before going crazy and stripping it out, I figured I had better see how the bottom two were since I couldn't fill it up until I drained out the old stuff. Got under there and the 5/16 key sank all the way into the abyss of both holes unlike the shallow fill location up top.
Not only did I have a "depth" problem - with no way and not enough room to work with anything longer due to the big plastic wheel well guard (do you have to remove this?), but the allen wrench just spun around so it isn't a 5/16 like the fill. So I am puzzled about several things.

Once and for all - what is the correct size and/or sizes for these three plugs? I searched all around to make sure I couldn't be confusing these two things on the bottom of the transmission and did not see anything else remotely close to a hex-key hole. The bottom most one - which I assume is the drain plug - is at the bottom of the tranny (drivers side) near the hole for the starter gear (which in my case is one of about 10 places that leak oil including the rear main seal), then the check plug is behind that closer to the axle maybe 6 inches toward the firewall and up just a touch about an inch higher. They both appear to be at the end of somewhat cylinder-like shaped steel. Am I correct?

Are all three really 3/8 and maybe I got thrown off by the 5/16 fitting the upper one (geeez I hope I didn't strip that thing).

Any help, guidance or suggestions would be appreciated. I would love a video for this - there's a video for practically everything else online out there if you search long enough.

Gave it another shot this afternoon with a 3/8 inch hex key - the same one I use for the brake caliper bolts. Again - no.

Can someone fill me in on what the secret is to getting these things off? I just don't get it. I cant see inside of the two bottom mystery locations
because of the angle my head is looking at them from - so that means I can't judge the size, or depth.

I also just flat out am confused about the fill plug - on top - seeming to be for-sure a 5/16, yet having the two bottom ones not the same size.

Anyone?