if you absolutely cannot loosen the screw on the linkage by hand, hit it with a heat gun. be very careful and start w light heat, and focus the heat only on that screw. it will melt wires and damage other things if done haphazardly. a few back and forth's between the heat gun, and attempting to turn it, should do the trick to get it started.

i think he's referring to the female hex nut that holds the linkage to the flipper shaft. you need to use an allen key.

also i'm pretty sure this is the proper post replacement, unless they used one with a shorter base to compensate for the flipper baseplate covering it.
https://www.pinballlife.com/2-14-tall-metal-post-with-threaded-base-and-female-threaded-top.html

you can't use a box end wrench. stern uses a socket head bolt that screws into threads on the vice. it's not a standard nut like used on BLY/WMS. you need an allen key (or apparently vice grips)
yikes (resized).jpg

yep. sorry for the bad terminology. but yeah, stern still does this setup. maybe for patent reasons? just my completely uneducated, out of the loop speculation. but i actually replace all the stern pawl/linkages with WPC once the factory one's give out. much for this reason.