Area 419 Zero Press vs SCA Nexus Press

it's on the bottom where tube goes into. it locks the shell holder carrier, so it can't be slide out.
That set screw is removed and replaced with the primer tube. The primer tube is then hand tightened to lock the shell holder and shell holder support disc.

The shell holder will have some play in it to allow it to float.
 
it's on the bottom where tube goes into. it locks the shell holder carrier, so it can't be slide out.
I had a similar thought about mine at first and used the drop tube to hold the shell holder in place. But, that's not the way to hold it in place. There are two removeable hard stop disks to insert at the back of the shell holder that holds it in place:
 
oh! I get it now, set screw was there for just shipping purpose, so it doesn't slide out.

I am not using the tube anyways, make sense when I put spare stopper (bumper) on the slot, that will stop from sliding out.

Thank You guys (y)
I just emailed them and they confirmed this. Primer drop tube or bumper. Set screw is just for shipping.