>>3255969 I don’t think this lens is the usual type of double Gaussian, because right after this element is the small and last doublet/triplet before the focal plane.
It’s interesting comparing this to the diagram for the Noctilux. Both are super fast lenses, but the design seems very different. The front element of this lens is positive Plano-convex whereas the noctilux is negative, similar to a wide angle.
I found this diagram digging deeper on the web. Pretty sure this is the left one, and the right is the 65mm f/.75 variant. I don’t know what the vD is — is it related to effective focal length? The left is Refractive index I think. In which case the doublet is two elements and each are 1.7ish, rather than my far off guess of 1.9 based upon what I read online elsewhere.
