Unusual Septic Drain Field Issue

[Stuck-in-the-Muck]

I found a concrete slab buried in my back yard where the drain field is supposed to be. It appears to be "L" shaped 22.5' X 21.5' on the long sides and 5' wide. It is about 8" deep, hollow with baffles. From the open ends I can see rocks inside, but have not gone too far into exploratory mode. It abuts the outlet to my septic tank where a previous owner demolished one corner of the slab (at the 90 deg in the "L") to make room for a modern distribution box. House was built in the early 60's.

I am not sure exactly what this is. Any thoughts?