Hi,
It's all trivia & fwiw, but I've definitely never seen it like this before.
It seems to be the stock TR25 12"speaker (with codes I've seen mentioned for the TR25 here before:
SP122016G) so likely it must have crossed the eye of various other TR25 owners as well.
At least assuming they unmounted the speaker to give it and the cab a proper cleaning. Alternatively, pulling off the velcro-attached speakergrill would already show it.
If it's not common though, then possibly it's a reconed Eminence, and everything is possible.
Just for the sake of trivia I'll show it to a friend as well, who has seen at least as many instrument speaker cones as I have.
While instrument speakers will be more forgiving than those for hi-fi, much thought seems to go into design of the former, to say the least (I recall an article in... SoundOnsound?, from a Celestion-designer).
Can well imagine that overlap-section has a drastic impact on the vibration pattern of this speaker. Who knows it's both cheaper & beneficial ....
Bye!
BTW, from your signature:
Epiphone Jack Casady --> yummie
Ovation Magnum 1 --> on wishlist

Peavey TB-Raxx --> nice to see this one being mentioned