The Willow-SE attachment appears to work with Extender as well
I was just checking things in the armour hall and it appears the Alpha Nine helmet's attachment 'Willow-SE' also works on the Extender helmet. I don't know if this is just a visual bug but I'd assume not. Assuming this isn't a bug then this is one of the few helmet attachments that works across helmets from different cores, with the only other one I'm aware of being Needle Minder.