It doesn't look like Solnhofen sublithographic limestone. The piece looks more like eocene material from the Green River formation in the US. The piece is a 'blown' fish replacement. Meaning the fish decomposed and partly disarticulated before the fossilization process could begin. A rough guess from the fin ray sizes and general body shape indicates the species to lean towards Priscacara sp.
Here's a Priscacara for comparison: