I think it’s this one:
http://www.akkordeonwerkstatt.ch/bausatz.htm
Babelfish says:
With our kit you build yourselves a Diatoni “Einsteiger Oergeli” in C major with a tonumfang of more than one oktave. You accompany your melodies with C and g-g-Quinten bass lateral.
They told me that for the 21-button, the buttons are smaller and closer together than standard, which I don’t know whether I could get used to.
BTW they also told me they had no experience of selling outside the EU when I asked whether the prices included sales taxes which they could knock off for export.
Here’s a human translation of the next paragraph:
For 120 Swiss franks we supply fully prepared materials, reed blocks ready for insertion, help and detailed instructions for building a miniature diatonic accordion. The instrument is kept as technically simple as possible, but is not a toy. It is suitable for accompanying children’s songs or for folk music.
Sweet! Thanks for your help, I’m going to try to send an inquiry to the company.
Ack, I’m not sure if I just managed to send them a message or not. Do any of you see a sign of an e-mail address for them on that web page?
The reply to the query I sent through the website came from info@akkordeonwerkstatt.ch
Mind you they didn’t reply to the answer I sent back to that address - there was no real need I suppose.
It might help if you wrote in German, you never know. Nothing on the site suggests that they speak English… 
Another French maker with the right inside/outside concept. Click here http://www.pianoabretelles.fr/Diatoniques_Stelvio.htm and select “models”. It’s the Kerry.
Many highly attractive custom accordions out there… judging only by the photos, the Untersee guys even appear to make their own bellows, which is pretty rare these days.
Thanks. I think maybe it’s time to break out my German-speaking, trad-loving secret weapon…