In my 602 before I rebuilt it, a gap developed between baffle and the back wall due to the baffle warping. That gap really messed up the burn, overheating the back and cracking it. The replacement baffle sealed well to the arch on the back. In this case, it looks like on the 606 there is no arch for the rear of the baffle to sit on due to the high flue outlet. I'd seal the gap as best as possible.
