Mamiya during the late 1960s had the well respcted TL and DTL SLRs with a M42 mount. Around 1970 however it was obvious that future professional cameras would have to have a bayonet mount, mainly to allow for easy open aperture metering (or even automatic exposure). Mamiya had produced an "auto DTL" prototype around 1970 (based on the DTL), but wisely switched to a bayonet mount when releasing the "auto XTL" camera in 1971. Apart from an Interchangeable viewfinder the new Mamiya auto XTL had everything (and more) a professional SLR needed at the time, including an automatic exposure (shutter priority), a switchable Spot / Integral Metering system with direct light metering on the mirror, and even an AE-Lock switch!

Sadly, the market didn't accept the new Mamiya bayonet mount, even though Mamyia was producing dedicated lenses with a focal length between 21mm and 800mm. Even though Mamiya tried to introduce a modified Auto X1000 in 1975, it was produced only for one year before the Sekor ES system was abandoned (and later replaced by the Sekor CS bayonet).