It's not something I've ever done myself, as I don't have any 144 pens. Obviously the push on pull off cap uses some sort of an internal clutch in order to stay on. It's possible that clutch might use the gold plated band as the pressure point for the clutch, but it won't use the nib itself..
Yes, they have a plastic inner cap. It’s not uncommon for them to split, making for a loose cap to barrel fit. The snow cap unscrews. The snow cap has a stud on it that extends down into top of the inner cap which has a threaded brass top to it. Once you get the snow cap off, the inner cap comes right out.
Roller balls also have a similar inner cap, look similar, but it is not interchangeable with the FP cap. Use caution buying parts.