Block Drain Plug for Coolant

I was under my truck tonight inspecting it and noticed a coolant leak, had a drip at bottom of bell housing. After further inspection noticed a little 5/16 square plug on driver side close to oil pan beside were manifold bolts to exhaust it was wet around it and appeared to be running down. All my books show block drains on passenger side around starter. It was dark so I didn't slide over and look but do some have it both sides or just one. I was afriad it was somthing worse but glad I found the leak or at least I hope. I got it to tighten 1/4-1/2 turn more. Any info.
Thanks