Removing stepper motor problem from sherline mill

Machine (new to me) Sherline 5400 with cnc

Hi. Despite lots of searches, YouTube, Google, forums etc I have to admit I am stumped.
Had the machine a few days (second hand) and had good results with Mach 3 demo and some example gcode.
Yesterday, the limit switch at the top of the z axis failed and the machine didn't stop on a "zero" command.
Steeper motor still turns but does not drive threaded rod. I guess something has stripped BUT.... Can't work out how to remove the stepper. I have been gentle with it so far but don't know if I should be using a"puller" to lift the motor with a bit more force but want to ask before I do.
Attachment 15130
Picture attached which I hope posts OK.
Attachment 15130
I am also intrigued by what the small brass assembly does, I hope it shows in the picture.
Hope I have given enough information.
Thanks

Solved..... My diabetic eyes couldn't see the tiny grub scree about an inch down the tiny hole in the housing. My daughter found it instantly after an email pointed me in the right direction........... Embarrassed!