I just tried 9V battery trick for 50’s Fender Champ 5F1 and Deluxe 5D3. and the speaker wiring turned out being reversed. both of them still have the factory wiring.
And, both of them move backward (to the magnets) when they wired 9V + to the white wire, and – pole to the black wire.
They mostly say there is no absolute speaker phase unless they are in the same cabinet. at the same time, many people say the speaker cones move forward when they hooked up in the right direction with 9V battery.
Fortunately, 9V battery didn’t kill my 8″ Champ speaker, nor 12″ Deluxe speaker. but I’ve read some cases this trick killed their speakers.
I’m not sure with 50’s and 60’s Jensen speaker’s correct phase. but, I started to believe Fender hooked them up this way. I’ve seen only 2 cases. and will update if I come across any Jensen speaker with different phase.