I have sent Gunter Hampel (*1937) a link to this topic. Here is his answer - with his permission:
"1964. Eric Dolphy was checking my vibraphone, I got bis bassclarinet on my request. I play clarinet since I was 11. After we both had a jam, he on my vibes, I on his bcl, Eric said, I should buy me one!
Some times later, 1966, I was playing with my quintet at the Blue Note in Paris, opposite the Kenny Clarke Quintet. They played one set, we played one set, 3 or 4 times each night. Kenny was holding drumworkshops at Selmer. They sold me a flute and a bassclarinet. I got the one, which goes to the low Bb, which I still play today.
On my recording "The 8th of July 1969" I pinned sections with two bassclarinets (me and Willem Breuker, who was playing in my band for over 6 years) + a contrabassclarinet (Anthony Braxton) + Jeanne Lee on voice and great Steve McCall on drums. A few years later I got from Selmer a contrabassclarinet from wood. Anthony's is from metal. If you like, you can go to "youtube" and hear and see a piece I performed solo in the Philharmonic Hall in Berlin on that huge horn. I used to play both bcl and contra-bcl a lot with the Galaxie Dream Band, in which I was featuring a string unit with violin, cello and two contrabasses and a clarinet group with two clarinets, alto-clarinet and bassclarinet. It is all documented on Birth Records + voice of Jeanne Lee a.o. ... and still available."