This pencil cartridge has the same size as the BP refill. It was intended for the BP and can easily convert a BP into a cap-actuated mechanical pencil (MP) instead of the normal early 75 pencil that used a twist mechanism to draw out the lead. The lead in this cartridge is quite thick at 0.9mm.
It is too bad people disposed these cartridges as they are now hard to find. It turns out that you could simply refill it by inserting the leads through the narrower point.
Later 75 MPs used a different source of the mechanism (Ohira of Japan) and used much thinner lead at 0.5mm.
Notice that the newer cartridge had the spring already pre-installed. If you wanted to use this in your 75 BP (or any other Parker BP like the Jotter), you would first need to remove the spring that was force-fitted in the barrel.
One warning ought to be mentioned. Converting a BP into MP using the Japanese cartridge was often thought to be a one-way conversion. This is because the cartridge had a split O-ring
that had to be shoved through the barrel opening. The purpose of this O-ring was to prevent the exposed tip from retracting into the barrel, but it made it difficult to remove the cartridge without damaging the tip of the barrel.