There is a lens described in a classified CIA briefing, which was never built [1]. The lens was designed by Shoroto Yoshido, a lens designer from the Sendai Science Laboratory, who supposedly achieved a relative aperture of f/0.519 using 5 elements. The lens only had one aspheric surface. But its design was somewhat crazy, the focal plane almost touches the the extremely curved rear lens. There is very little else written about this lens.

Further reading:

1. Merigold, P.A., “Aspheric Optical Systems”, CIA Reading Room (released Nov.21, 2005)