This sure sounds like a rainwater cistern to me, but without tracing the piping, it is impossible to tell. My guess is that the 1-1/2" pipe is connected to a downspout to collect the water from the roof. The rest of the piping is an overflow system to get excess water to a weep field. But I stress that you don't know for sure unless you follow the piping.
You might check with some of the plumbing supply houses in your area to see if anyone remembers or has seen similar systems. It doesn't seem surprising that it is not on any maps since I don't think that you would have had to pull a permit for a rainwater cistern in 1970 in most locations any more that you would have to for a french drain today.