Well I'll be!
There are a couple of my scopes I'll be having a close look at. I seem to recall an objective or corrector plate having a similar uneven edge. I had noticed an unexplained diffraction spike on one of my scopes that doesn't have a spider, so now I have a possible source.
Out of your pain, Kuz, this is a really good example of why a field stops & baffle rings need to be perfectly smooth and kink free, and focuser drawtubes not cut into the optical train.
I wonder now how many Newts have unexplained diffraction spikes or flares due to the drawtube? And have the scope's owner tortured not knowing the source. Especially with so many Newts needing to have the focuser rucked in just about all the way in to achieve focus with a camera. I have noticed the shadow of a focuser draw tube with one of my Newts (a solid tube OTA) when the focuser is rucked in al the way. Time for a closer look...
Excellent investigative work, Kuz!
One to certainly keep in the memory bank of problem solving.
Alex.