Canon A-1 - Manual (Page 143)

All FD lenses which lack a Breech-
lock mount ring, with the exception
of the Macro lenses, are set for manual
diaphragm control as follows:
1. Before mounting the lens, insert the hole
of the accessory manual diaphragm adapt­
er over the tip of the automatic aperture
ture lever at the rear of the lens. Push
the lever to the right and lower the
adapter into the groove to lock the lever
in that position.
2. Mount the lens onto the accessory. The
diaphragm will now open and close as
the aperture ring is rotated.
When the manuctl diaphragm adapter is
attached on the rear of one of these lenses,
never mount the lens directly on the camera
or directly on accessories designed for
automatic diaphragm control, such as the
Auto Bellows or Bellows FL.