They may have felt the necessity to have a 4mm fixed sleeve model that was the standard for drafting tables and drafting templates.
A 4mm metal pipe moves consistently along the edges of rulers and templates, at the same distance and at the same thickness and there is no contact between the lead and the rulers. With the 2mm pencils that many people used for drafting (architects etc) it’s the lead that is put against the edges and several things can happen:
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As the lead wears out the line gets thicker and the distance to the edge of the ruler is not the same — that induces minimal differences, maybe not so many on H leads, but it is possible that you’ll get some inconsistencies in a 2 or 3 meter long drawing if you’re using a 2mm drafting pencil. Many times, drafters would have to change their hand position so that the tip of the lead would meet the corner between the edge of the ruler and the surface of the paper but even so they’d have to be always sharpening the lead.
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Leads will leave residue on the edges of rulers/templates and that eventually will stain the paper.
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In smaller lettering templates 2mm will not work
I think this can explain the Fixpencil 75 with the sanded grip. It is a drafting pencil — the other 05 are general writing/sketching pocket pencils.
edited: lots of typos!!