I think the mark is German

[Babsy]

But to tell you the truth I am stumped. The word on the back is "CESTLIEN" the L is different than the other letters, much more stylized and I think this is a German letter and could be a "B"
I am sure one of you will know.

[Bahner]

Hello, for a German mark this is very unusual. I agree that the letter "l" is rather a "B". In that case it reads "c'est bien", which in French means "this is good". Best wishes, Bahner