I have all three, ET-65, Reaper & Invader.
I'm not a Celestion scholar.
But from what I've deduced Invader is a HP 12M. But so is ET-65. A higher power 12M.
And a Reaper HP is a 30 W Green Beret. In other words. a 12M turned higher power.
It all seems very redundant. But the tones are obviously different.
Sounds:
ET-65: Sterile without color; loaded with Mids.
Invader: Bright with top end wide open.
Reaper HP: quieter than Invader, smoother.
Historically I've posted like a moron before. I hope some of this makes sense. I sense my sound interpretations may be way off.