Here is YET ANOTHER example for your consideration. . But I don't think it is a match for your pen kit.
It is the Berea Cigar Pen kit ..... here is the link :-
https://www.bereahardwoods.com/pen-kit/sets/cigar-pen-chrome.html
I have done a lot of work with this particular pen kit. . But for some reason I had a mental lapse until a few minutes ago.
It has similar architecture to yours ..... twist mech screws on in a similar way.
Only problem is that this kit has bigger thread OD ..... measures at 7.3 mm, so I suspect the spec is actually M7.5 x 0.5 mm
Twist mech has female threads; coupler piece has the male threads (which I think is the same as yours).
So, while not fully compatible with yours it may be an example to keep in the back of your mind.
Many companies sell this style of pen kit (some "better" than others) and you should be able to find in the UK if you should ever want to.
I really like the general features of this pen kit styling. . The Berea model which I have referenced is probably the best available for smooth operation and reliable plating options.
I hope that some of the info I have provided in the above 3 posts will be interesting and/or useful to you.
Please keep in touch and let me know what you come up with for your project .... also feel free to send along any questions you may have.