Thanks guys.
The last one was a relic of the dodgy 4-2-1 that the previous owner tried to make. I re-made the 4-2-1 but kept the old ZX9 flange. Then when I switched over to the ZX9 4-1 headers, I already had a muffler to suit the flange so I kept it. But I finally bit the bullet and decided to do things properly. So a new link pipe was fabricated and converted to a cleaner looking slip-on. It feels the same, I doubt there is even the slightest difference between any mufflers that are un-baffled, and it previously has done 135-ish HP with these headers. It sounds great with a deep bark but not too offensive even without the baffle in.
No chance of making another set
I am more starting the think that maybe the Z750 headers and pipe would have made the whole process much easier. At least it already fits the same bike. Still, it is good to have a one-off!