It would help to see photos of the removed barrel.
Depending on the vintage of the pen, it may or may not take a "standard" converter, unfortunately. I'm finding that out with the Skripsert pens -- apparently the modern Sheaffer twist converters do NOT fit in them correctly.
Are you certain that it even takes a cartridge or converter? Because at a quick glance, I would have thought, from the Triumph nib and the general shape of the pen, that it might be a Snorkel or a Touchdown. And not being able to see the back of the pen because of the cap doesn't help. More pictures would be helpful (I'm not a Sheaffer expert by any stretch of the imagination).
Ruth Morrisson aka inkstainedruth
Not a 50s Touchdown or Snorkel... The only full metal body models of those (per my references) were solid gold ($100 in 1952!). All the other models had plastic barrels. Biggest give-away is the clip.
Squared clip appeared on the PFM, but again, those had plastic barrels. My sources don't show any brushed steel models until the Imperial line, starting in the 60s which accepted cartridges, had the squared/spring-loaded clip (and based upon the one I own, was available with Triumph nib in some variations, even though the norm was the inlaid nib). Those lasted into the 70s, being phased out by the Targa.
The Triumph Imperials where a mid-90s recreation, and misleadingly, had inlaid nibs... The Crest had a Triumph nib (but rounded ends a la Touchdown/Snorkel and narrow clip...)