As Ludwig. already wrote: Elmar had basically built a few ukes for friends and family. His brother, who also made the few videos with Ukulele Uff etc, is quite a versatile guitar and ukulele player. And he brought two or three ukes for sale to a meeting in the Netherlands. Elmar obviously has a talent for building wonderful ukes. As far as I know, he has some craft background. His instruments are amazing, very light-weighted, very resonating, wonderful tone.
But it‘s not surprising you can‘t find any information. I don‘t think he made more than ten ukes in total, if at all. And he didn‘t make any after the one wwelti bought and uses in some videos. I‘m still hopping mad about myself, as the I had offered wwelti‘s uke before him, but turned it down. If I had known that it was the last Rubbenstroth uke for the time being... lesson learned!