Pencils, hmmmmm only get enquiries when I don't have any!!
The problem with selling pencils is that their intended use is specific, be it woodwork, artist drawing, or crosswords. Mechanisms for leads of less than 0.9mm are about as useful as a chocolate teapot. The leads are so fragile that they repeatable break. The cigar pencil is one of the most reliable but that isn't saying much, I don't sell them as they aren't commercially viable in that the return rate is far too high. The type of mechanism that goes into this kit is ok, the interface with the pen is what lets it down. Never made any sierra pencils so can't comment on these. The timberbits retro thing is appalling you are advised to carve bits off it to fit some pens. The only pencils that I sell because they work are the vertex and the wood workers pencil, the first has a 2mm lead, the second a 5,6mm kit both with a clutch mechanism. They both work well, I have not had any returns from these pencils, the only issue that frightens people off is the perceived availability of leads. Not tried the stratus from psi but it uses the same mechanism as the vertex.
Sure there are probably more but these are the ones I have tried.
I cannot see why someone would want to buy a pencil in the shape, or more importantly the weight of a Zeta. The pen is designed to have some weight to assist the flow of the ball point. You certainly wouldn't use a heavy pencil for artist sketching, and you wouldn't use it for woodwork, and in the absence of any other specific uses, consign this one to be used for crosswords. Any pencil that is a retrofit is a compromise. I will steer well clear of any retro kit.
I bought fifty of the pencil conversion things, I have about 43 left!
