Fafnir was different from Alberich. It consisted of a skin of rubber forming lots of small prismatic rubber teeth (like the egg cartons used in some studios for sound insulation). These were held to the hull by sheets of plastic in metal frames, which were in turn attached to the hull. The whole construction was very flimsy and only used for extensive testing in 1940. The metal frames are very visible on the hull in the one photo that I have.
Alberich as used in U 1105 and a few others consisted of pieces of rubber skin (without teeth) glued directly onto the hull. There is one detail photograph of the Balkongerät
which is is actually of U 1105, and you can see the seams between the pieces of the Alberich skin. Cannot find it in my files, unfortunately.