The lazy susan doesnt appear to be a modification. Looking at wear marks, I don't see any from a previous drawer. The hardware is not original, but I'm sure you all noticed that.
So, if the lazy susan is a mod, then the wood supporting it is modified also. That sheet of plywood matches the interior of the cabinet. Its really strange, I'm not to sure what to think of it. I sure like it. In its current condition, I see no real value to it, so I may re-veneer it.