This is one which foxed me.
The nib and the banding on the barrel and cap reminded me of a Vacumatic.
The cap band looked like that from a 1950s English Duofold.
But shape, tassie and the fact it was a cartridge filler tended to push me towards some sort of prototype Sonnet.
Further enquiries revealed that there were no markings on the barrel and the only wording on the nib was Special.
Any ideas anyone? Is it a Parker?
Ebay Parker