As for design, not sure. It is a round pit 12 feet wide and 13 feet deep. Does that give you a hint as to the design? The septic company guesses it is cinder block surrounded by rock. We have no idea if there is a clean-out, but the septic company did offer to dig it up and pump the pit. Based on the depth of the septic tank top, it is estimated that the pit is 5-6 ft under the surface--they want to bring in a backhoe.
As for lost baffles, I don't know--I'll ask.
As for water run back, it was way more then a a few gallons. It ran back at a fast rate for 10-15 minutes and was still coming in slowly after they finished pumping the tank and put the cover back on.
Thanks for your help here. Any guidance is appreciated.