I received now Doehring's Ph.D. thesis (1953, Braunschweig) from Arend. 
It appears that the article from 1955 is directly based on this thesis. 
Still, no explicit comparison with previous results is made. 
Some errors in the thesis are corrected in the article, and some of the 
notation has changed.  There is a reference to Grammel's book, 'Kreisel'. 
Should this be included in our literature review? I do not have quick access 
to this book, but Arend might have. (1st edition 1920, 2nd 1950). 
Please, have a look, Arend. There is also a reference to a certain Galetti. 
I have no idea who Galetti was or what he has done. 

JPM, Nottingham, 8 Nov. 2005.