My guess is that it was a bought in line to expand their main business that probably died in the years of inflation, as there is not enough branding on it to be used for promoting the company name.
The inscription looks like a personalisation: Frank Swan to me, and the pen one of the numerous British Duofoldesque pens produced in the 1930's. Probably produced by Mentmore, Langs or Valentine's.
I think the nib is a bit of a red herring as they were major replacement nib producers (prior to taking over the Swan stock)
Isn't 143.5 mm about 10 mm longer than the streamline? the celluloid (red version of jade?), clip and cap band suggest a very early streamline using Duofold parts while the nib (replacement?) and section appear to be later Duofold items.
It must be said that just about all British pen makers copied the Duofold for their button fillers. Have you checked the threads against other Parker pens?
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