The part that protruded down from the cap along the side of the pen to form the “pocket clip” fit between the gear teeth and made turning easier & more reliable - you slid the cassette down the body of the pen until that part engaged the teeth. Later I came to prefer the style of pen below due to the flatter protrusion, but both worked pretty well.
Japanese pencils are slightly bigger in diameter than American ones, they fit perfectly into the cassette sprocket while American ones leave enough room for the pencil to spin without spooling the tape.
Bic pens were better because their caps were shaped perfectly for the job.
I dont know which cap you had but it was the worst idea because my caps were round and unsuitable for that. I used the pen itself.
The part that protruded down from the cap along the side of the pen to form the “pocket clip” fit between the gear teeth and made turning easier & more reliable - you slid the cassette down the body of the pen until that part engaged the teeth. Later I came to prefer the style of pen below due to the flatter protrusion, but both worked pretty well.
Japanese pencils are slightly bigger in diameter than American ones, they fit perfectly into the cassette sprocket while American ones leave enough room for the pencil to spin without spooling the tape.
Philips designed the compact cassette specifically to be wound by a Bic pen.