Hi,
I agree that it is likely to be a Hanau item. The Netherlands is still a possibility, but although the Dutch also used these antique fantasy pseudomarks, in my experience, they had a tendency to include one or more officially sanctioned marks somewhere on the piece. They did use a dolphin mark similar to this one, but not quite this one.
See ->
Netherlands Hallmarks
There is a good, 2 part, article on Hanau silver by Dorothea Burstyn located on the website of
ASCAS
Regards, Tom