Your first assumption was correct, Heise is the assayer, FIH the maker. I've just checked with the Danish Ã†delmetalkontrollen, and they don't have an FIH registered anywhere near that timeframe, so I think it is likely to be Hingelberg.
The images should show up, maybe that free server where I put them on was down for a while. Any clues about that hardly recognizable mark at the upper left, or the mark with the little mouse in the centre???