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Thats pretty much about it. Wearever cartridges havent been made in 50 years or so & no modern ones will properly fit. If your dead set on using a cartridge, you'll have to track one down & refill it with a syringe. (you can get allergy syringes at nearly any pharmacy). Often times you'll find NOS blister packs of cartridges in ebay for $5-10 or so.

OR you can use a small schmear of silicon grease on the section threads & fill the pen like an eye dropper filler. I personally prefer to go this direction because the pen will hold a bucket of ink.

The Pennant isn't the only model that can be used as an ED like this. Prety much any Wearever cartridge pen can be converted. The Sheaffer no Nonsense and school pens are great for this as well. heck, for that matter just about any threadded section & barrel that is sealed (no air/vent holes in it) can be used like this.