RE: OS FS-70 Venting - What's correct way?
It sounds like the previous owner modified it for smoke. A picture would be helpful in helping you understand what you have.
On a side note, are you sure you bought an FS-70? I'm thinking the FS series all had metal backplates, unless it was damaged in a crash and replaced with a plastic one. Is it maybe the FL-70 you have aquired?