For completeness I added the Trezor key (dropbox lets you have multiple security keys attached to your account), then removed the Ledger Key... logged out and tried to log back in with the Ledger Key... and got the same message. So, DropBox sees them as 2 unique keys.
Based on this quick and hardly thorough test, it would seem to suggest that the U2F is
not cross-compatible between different
brands of device, even with the same seed.
Probably the U2F protocol implies that the technical characteristics of the U2F key (its brand and manufacturer) must be the same in order to satisfy the protocol. And this is entirely logical, because it is thanks to this that increased security is achieved when using these keys.
It would probably be surprising if you could recover your key using a seed phrase using any device of this kind produced by different manufacturers. The fact that the dropbox sees these keys as if they are different is our additional guarantee in case someone becomes the owner of our seed phrase and wants to restore the key on another device without knowing about this feature.
I wonder if there are any exceptions to this rule? Maybe among the whole variety of keys from different manufacturers, there are still some that would be interchangeable with respect to each other.
It would be interesting if someone did such a large-scale study.
By the way, I recently had a situation where I was trying to register my Trezor One as a two-factor authentication key for my new email on gmail.com.
I don't remember exactly what kind of browser it was - either safari or chrome, but during the registration process, a window popped up that Google wanted to scan my key type. And I refused to give Google this right by clicking the corresponding button.
And to my surprise, after this, i could not complete the registration of my Trezor as a two-factor authentication key. Perhaps precisely because I refused to provide information about the parameters of my device.
I don't know - maybe it was some kind of exception to the rule, or maybe it's some kind of new Google policy regarding authentication keys, because earlier, as far as I remember - about two months ago, when I registered my Trezor in this way - everything was ok.