Yea, I think that's supposed to have a plug in it.

Thanks for posting those pictures.
Like I said earlier, those 2 housings you put fluid into should have 80-90 weight gear oil, not hydraulic oil.
Does that hole in the front case (TA housing) feel like the threads are stripped? I hope not, because I don't know why someone would just take that plug and leave it out. Weird.
I don't know if that plug would be NPT thread or pipe thread, you might be on your own for that one. I would guess pipe thread but I'm not sure.
If you wanted to play it safe, there most likely salvage yards that would have that plug for cheap, I don't know where you are located though. If you wouldn't mind sharing your approximate location, I'm sure someone could chime in and find a yard near you.
Well, if you are confused or have any other questions please ask, otherwise good luck!