The best option in my experience is to make make a mandrel with the correct thread just slightly larger than the inner cap diameter and then with a hair-drier heat the cap and start screwing the mandrel gently ( the mandrel it self has a very slight taper shape) Several heat followed by screwing in an out are needed
Instead of a custom made andrel if you have more barrels from a similar pen but just a tad larger you can use them also. It´s never 100% perfect ( In my experience) but it works quit good.
Just be careful with the heat, to much and you have a ruined cap...
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Link for Vintage Montblanc pens here
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