Two possibilities come to mind immediately for that aperture and focal length.
1) It's for a extremely high resolution lens, with absolutely no chromatic aberration or distortion. Something like the Zeiss Superachromatic designs of the 1960s. Many say their sharpness has never been matched and give stunning results in black and white and in color (although it's too sharp and unforgiving for flattering portraits). Can also be used for IR photography as the focus is the same across a wide range of wavelengths. Notable for it's six element design with one achromatic pair.
They were limited-run productions and ferociously expensive because they had fluorite elements and they actually finished the lens design after they'd made the crystals and could measure the optical properties properly.
Maybe Canon have found a way to make an achromatic without the cost of the fluorite elements.
2) The other thought is that it's for a medical lens with a ring flash incorporated. This is a Medical-Nikkor from the 1960s in 200mm f5.6.
They were used by dentists, cosmetic surgeons and dermatologists. Especially useful for close-up shots without being too much in the patients' face.