The end stop cap compression fits into the square hole to allow for the X end stop to activate. I added the washers to match my colour scheme as the red didn't match with anything on the printer.
Link to the Funssor Ratrig V-core4 3D printer Aluminum CNC ultra weight light XY joint kit for VC 4.0 XY joiner upgrade set:
I haven't tested it, but it should also work with the same product from Blurolls:
The author marked this model as their own original creation.