That's good news Marc!
As I recall it is rust.
I'm not so sure I'd call it "rust." "Rust" is what occurred with the Coronado Induced Transmission Filter (ITF) coating, which is an IR blocking coating using metallic silver and other materials to block to the far IR. Thermal cycling caused cracks in the filter's epoxy sealant around the edge, which allowed atmospheric moisture to degrade the coating, and therefor deterioration usually proceeded from the edge inward.
This appears different with deterioration towards the center of the filter. I'm not absolutely sure of this, but I believe Lunt has switched to a non-metallic/silver IR blocker - perhaps a KG heat absorbing filter or other material, which has AR coatings. Andy Lunt stated it is the AR coating on these filters that failed - due to processing deficiencies form their filter coater. This occurred with the early filters, and they now have changed filter coaters.
As you could imagine, additional requirements might be needed for the intended use of direct solar thermal exposure. So while the cause might be the same - thermal exposure - the Lunt issue appears to be coating degradation due to direct expsure to thermal energy, while the "rust" of the Coronado ITF was due to moisture deterioration.