I've picked up a huge old Sigma macro zoom that was designed to use their old interchangeable YS mounts - basically a T-mount with a stop-down pin. Unfortunately the mount is missing, and the lens doesn't offer a manual stopdown setting, though I can probably fake it by putting on a T-mount then wedging something between the mount and the lens's own stopdown pin. But a proper YS mount would be preferable. On the whole I'd prefer M42 because I have a good Eos-M42 adapter, but anything would do, although I'd prefer to avoid anything that was built for open-aperture metering such as Canon FD - YS mounts for open aperture were allegedly very unreliable. If I still had access to a good workshop I'd probably try to build one, most likely as a T2 mount modification, but currently that isn't possible. Anyone got anything?