I'm still kickin' rocks 😑 but kickin' um here, & tryin' to smile 😬
The piston assembly will lay loose in the slots until you lock everything down with the steam chest.
This is another loco with a similar set up, but on the engineers side instead. (665) The dot in front of the bolt head is the end of the piston rocker arm (stepped bend) and moves fore and aft , piston and gravity(&/or spring on piston) keep it resting on the crosshead. (the spring seen is so if the piston hangs up, it doesn't bind up all the way to the motor, and softens compression and shock in side thrusts too.