snip

The house is old construction, 50 years old, and has the original
septic system (which was pronounced in good shape by the guys who
pumped it out last spring.) So my guess is that maps, plans, etc as
well as the original plumber or septic company are loooong since gone.
snip

In our area, the health department keeps record of septic system
installations, even old ones. Worth a phone call...

Paul