Gift from an another friend, also got the second MD 50/1.7 & MD 28/2.8 too

This lens is not easy to use, you have to press the MC bottom to close down,
(with a normal film camera you do not need to press MC botton of course)
but releasing it wont come back to open aperture, you have to set F3.5 to open.
(mine has too sticky blades?, or normal behavior at that era 1966~1969)