I was away for three weeks from the forum, due to a trip to the USA and a lot of work on the book about the Mauser Parabellum pistols that I want to finish now very soon.
During the evolution of my research I started using C/c/U, or Cc/U as the shortcut for the Oberndorf pressure test stamp. As Martin explaned above, this is the correct way to describe the Crown above crown above U of this marking. See picture below.
While working on the book and needing Tables having a column for the proof marking, I needed to adopt the format to other proof stamps, such as C/N and E/N. The Cc/U was truncated to C/U - just to shrink the colomn width. I do apologize...
Joop van de Kant, co - author of:
"The Mauser Parabellum 1930-1946"
Analysis of a Million Luger Pistols