Now that is not exactly what you may need. But when I went to the Jetpens site, it suggested this:
https://www.jetpens.com/JetPens-Ink-Syringe/pd/22182
When you have a spent disposable cartridge, you could rinse it out, and refill it with whatever ink you wanted using this syringe.
But yeah, the OEM converters are pretty precious.
Saul
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www.newpentrace.net