Key 1 turns about fixed axis B and has pin a which slides along slot b of lever 2. Lever 2 turns about fixed axis A and has pin c entering slot d of link 3 which turns about fixed axis D. When key 1 is depressed, bent lever 2, turning about axis A, turns lever 3 to the position shown by dash lines. Spring 4 returns key 1 to the initial position.
$1398$LG,K$