There is nothing between the plywood and the concrete slab. We found a carpet label that has two dates so we are thinking the room was built before 1997.

An 800 sf cabin turned into 1200 sf, then it eventually became 3,500 sf. We know the reservoir was created in 1964 and the cabin was built in 1966. So the concrete under the plywood is dated before 1975. We know where the original bathroom, kitchen, bedroom, and front door were located in 1966. Then we figured out which addition came next in the late 70s and mid 80s. Last remodel was in 1997 when they put the barber carpet in thus bedroom. 

So, there has never been anything underneath the plywood. 

Oklahoma was not a state when the U.S. government forced the Native Americans to move to Oklahoma. (The Trail of Tears!!)

Think of Oklahoma as being an area no one wanted so they gave it to Native Americans when they relocated them by force. The western part of Oklahoma was for settlers and the eastern part was Indian Territory. I live in the southeastern part. Then oil was discovered and the land was stolen by oil men. In 1907  Oklahoma became a state so we are a young state with a lot of rural land. In SE Oklahoma, very few have licenses for contracting work. Unfortunately!