Leaking Transmission And Missing Bolt

I've had the car since September last year and I noticed this spring that I've got some transmission fluid leaking so I climbed under the car but I cannot really tell where it is coming from. I recently installed a Hurst shifter and thought maybe I didn't seal it well enough but everything looks clean. I noticed there appears to be a bolt missing from the transmission. Does anyone have any idea what size this is or where I can find a replacement?

Also, any thoughts on where this leak is coming from? I realize the picture is not great, I didn't have much space under the car to take it.

*Oh, supposedly the transmission was rebuilt shortly before I purchased it in 09/2016. Beyond the leaking and slight grinding when shifting into first after coming to a stop i haven'I had any real issues with it.

Thank you. I looked at some photos and videos of the transmission removed from the car and noticed that the top two bolts attaching the tail housing to the main case are directly in the fluid cavity so I'm afraid one of those may be leaking since it seems to be dripping from up above. It's either that or the top cover. Either way, I suspect I'll have to drop the transmission to correct the leak? I'm going to degrease, top off, and try to find a replacement bolt.