Update 5 I win
after a couple more soaks and a little help from a rubber tipped dental pick I have defeated the mystery stain!
Also I think I forgot to mention that this was my daily driver combustion piece for about 2 years until I saw the light, but to all of those who say a piece that was used for combustion will always have an off taste and not be "pure for vapor" hasn't tried this cleaning method.
*Protip #1 for those that want to do longer soaks and not get rings I did 3-4 6+ hour soaks and never got the PBW rings (although I know what you are talking about we leave carboys full of PBW for weeks and it happens) I was using pretty high concentrations but only filling to just cover the ice pinches on this piece so about 1/4 full YMMV.
**Protip #2 If you do get one of the dreaded PBW rings and don't want to soak with more PBW to get rid of it any acid should take care of it since PBW is an alkali cleanser I've used star san and plain old distilled vinegar with about the same level of success let it soak or wipe / scrub it off. Also useful as a rinsing agent for those who want to insure all PBW residue has been removed from the item being cleaned (should not be an issue with a proper rinse)
Protip edit
Got my SCS nice and clean too