Canon A-1 - Manual (Page 144)

All FD lenses with a chrome Breech-
lock mount ring and FD Macro lenses
are set for manual diaphragm control
as follows:
1. Before mounting the lens, push the
automatic aperture lever at the rear of
the lens to the right where it auto­
matically locks.
2. Moimt the lens onto the accessory as
usual. The diaphragm will now open and
close as the aperture ring is rotated.
Some of these lenses have an additional lock
lever.
With these lenses, the automatic
aperture lever must be pushed fully to the
right and the lock lever pushed to “L” to
hold the automatic aperture lever in that
position.