I think that is a Merrill SPK?
The disc holds 1/2 of the pitless that is on the drop pipe at the correct depth to meet the other pitless 1/2 that is on the casing. The rod with the broken nut adjusts a wedge that holds the 2 pitless halls together to create a seal. You will need someone to weld another nut to the rod before you can loosen the pitless up.
It might be holding steel pipe, but maybe it isn't.
I have never used one of these, but I can see where it could be a real pain in the ass