Recently a customer asked for help with a pen she had bought from me, a Swan 3120, the smallest Swan of its time. These pens have screw-in sections. When restoring them it takes a little fiddling to ensure that the sac doesn’t become twisted.
The customer had used the pen for ten months without any problem, then it didn’t take a proper fill anymore. It would only write for a few lines. I asked her to send it back and, sure enough, the sac was twisted. One can speculate about how the sac became twisted after ten months.
In any case, I fitted a no. 14 sac rather than the usual no. 16 one to ensure that there would be no repeat of the twisting problem.
There are situations where a screw-in section makes sense. A lever fill pen is not one of them. I’ve never understood why Mabie Todd made this decision when friction-fit sections had worked perfectly well for decades and made sac replacement so much simpler.
It was around the same time that they installed brass threads on the pens, an equally bad design. The brass thread on the barrel wears out the plastic thread on the cap.
Heaven forfend that I should criticise Mabie Todd, the finest pen maker of all (in my opinion) but the designers and planners came up with some weird ideas for a year or two. It’s not to say that we can’t make those pens work. It just adds some time-consuming and annoying work-arounds to the process.
Goodwriterspens's Blog 
I have a brown MT with brass threads on the barrel and the contrast between the two is beautiful. Not practical but I completely understand the choice from an aesthetic point of view.
I think the brass threads may be excusable from an aesthetic point of view but practically they make the pen less robust. The brass threads have an unfortunate habit of jamming in the cap. The barrel unscrews from the section and you are left with nothing to grip except the nipple which breaks easily.