The special hub is needed for all Birdy with front disc brake on the right side of the fork.

Rim brakes Birdy use a standard OLD 100mm front hub.

Nowadays, Riese & Müller mount a Shutter Precision hub dynamo. Previously, there was a SON hub dynamo for Birdy disc but as far as I know, it is discontinued (not on the website of SON Nabendynamo anymore).

As said, the front hub has a standard 100mm OLD. The problem is that the disc isn't placed at the same distance from the fork side (and of the center of the wheel) as with a standard 100mm OLD front disc hub, so a standard 100mm OLD front hub flipped to put the disc on the right side won't fit.