Does anyone know it?

I found one list at http://www.a1.nl/phomepag/markerink/mounts.htm, but it must be a little old and doesn't have the XL mount.

It's about 36 mm. This was deduced by looking up the back focus distance of a Nikon lens and subtracting the thickness of the XL2 Nikon lens adapter from it.

Ah, thanks... great thinking!