Yes, that is a syringe filler.
The tricky part is to find an o-ring that would match the dimensions of the original seal. Although now that I think of it, a seal punched out of synthetic cork may work better.
For the most part, these vintage syringe fillers have brittle or weak threading on the blind cap, and they don't survive restorations.
The term 3rd-tier is commonly used to imply lower budget production. I have no issue with the actual definition or the usefulness of the term, because of course, there are vintage pens that are made cheaply, and therefore the build quality is poor.
But just as often, the term is being used as a really broad brush, so much so that some pen people I know feels "embarrased" in liking a particularly good pen, just because some collectors decided that the brand "belongs" to the 3rd-tier group.
It's good that you have no such issue. There are some really excellent pens are out there for us to enjoy, just educate ourselves to separate the truly junk vs good ones.
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