I don't know anything about German pickled white fish sandwiches (Fischbroetchen) but your mention of them made me curious. I looked it up on the net and there are many variations of the sandwich but most are made with pickled herring or cod although you can also use other types of fish. Since you like them so much, James, have you ever tried to make it yourself at home? I know it wouldn't be the same, but sometimes close is good enough when you have a craving. Here are a few links I found. Take a look. Maybe one of them is close to the ones you remember.
This website is run by a pretty German lady who shares German recipes. She has a Fischbroetchen recipe for German Cod sandwiches.
[
dirndlkitchen.com]
Pickled Herring sandwich, German style - [
www.earthfoodandfire.com]
[
alltastesgerman.com]