German POWs in Gibraltar 1914

[ZavattoniP]

A couple of years ago I purchased this postcard written by a German officer from the prison camp in Gibraltar.
There are four censorship stamps:
one from Gibraltar and two P.C. I believe from Great Britain; my question concerns the fourth, the oval one.
I assume it comes from the prison camp in Gibraltar, is this a correct assumption?
Thanks in advance for any help.