Never seen one on a Mk-I. They're famous for getting the motor spindle bent after having the heavy platter bumping the pulley. The safety post was added later to prevent that. AFAIK It did not exist on the Mk-I. Maybe added on later serials. Are you sure you have a Mk-I? Any plastic parts on the bottom? Whatever... It's just an elongated cylinder with a screw on the bottom. It could have also been added by a previous owner. That's easy enough to do.
I didn't mean transit screws. I meant what I said. A sub-platter latch that retains a plastic disk around the bearing and prevents the sub-platter from falling off. The Mk-I did not have it. The latch is visible on the OP's sub-chassis.
EDIT: Here, it looks like this:
Sorry, I meant the bearing shaft (the part of the spindle that's inside the bearing well) not the top part. The cue lift is kosher. For some reason it looked weird and gray on a previous picture. Just an illusion. Now with the sub-platter latch thing being dealt with, you're going to have to release it if you want to take the sub-platter out of the well and get to the bearing shaft.