Hello! Yes, as orbitur mentioned, Apple tells you the device type after you enter a UDID. The one that didn't work was accepted, but didn't return a device type. We tried it anyway, but the app wouldn't install, which is why I described it as 'invalid'.

> Note that UDIDs beginning with 32-bits of F are returned by the APIs that provide app-specific advertising identifiers on the device, so if you saw one of those and decided it was "invalid", that was returned because they used a tool which tried to return a UDID but was using the older public APIs to do that from a sandboxed application

It did have a bunch of Fs at the beginning, so that is probably the explanation! Thanks :)