The term 'Pocket Pen' is slightly confusing as, it could be argued, that any pen with a clip is designed to fasten to a pocket so is, almost by definition, a pocket pen.
This is not merely a semantic issue though. The fact is that many pens were much longer than the average pocket and a true 'pocket sized pen' was likely to be too short to appeal to a wide market.
The ingenious solution, pioneered by the Japanese company, Pilot, was to make a pen with a very long cap and section, which would 'overlap' when capped, and a short barrel for posting the pen for use. Another Japanese company, Platinum, jumped in quickly with their version:
Pilot clearly had faith in their pocket pen offering, it was fitted with an 18 Ct. Gold nib with at least eight different profiles, this one is a manifold, extra fine.