Hi - hopefully we have someone here we can help - but if not then you might try the V. & A. Museum in London, as I know they do have some of his glass, plus they will probably have the book on Hadeland glass.
Their email address is............ ceramicsandglassatvam.ac.uk ... replacing the 'at' with the rat's tail.
The Corning also have some his glass, so would imagine they should be able to help.
But fingers crossed you'll get the answer here first.
