the rock has obviously been taken from the Late Triassic Megalodonta Limestone ("Kuhtritt" = cow step limestone). It is a very widespread facies rife with fossils mainly the bivalves which were used as namesake for the denomination of the calcareous rocks.
Yes, many of these are sections of bivalves. Several appear to have sectioned a very thick hinge area indicating one of the large Mesozoic oysters or perhaps Isognomonidae.
Madani's observation suggests me a possible difference between the rock of the two photos and the Dachstein Limestone of the Northern Calcareous Alps.
While the Dachstein Limestone shows the Megalodon bivalves mostly in-situ, fossilised in the position they lived and grew, this rock could be generated by resedimenting a bivalve-rich calcareous sediment, resulting in a graded bed, with a base (right) rich in bivalve shell, and a top (left) with much less bivalves. This would also explain the relative scarcity of connected and articulated shells.
The hand specimen is likely limestone and is full of shell fragments (bivalves), but most probably it will contain other microfossils too. It is better to prepare good thin sections for microfossil analysis.
The thickness and size and irregular sets is typical of oysters, but in my experience this type of oyster/bivalve is more typical of Cretaceous sediments. It appears the rock is not in place but floating in the rock slope.
I suggest it will be much better if you prepare thin sections of this rock unit in order to check its fossil content and to reconstruct of precise age. Best regards.
Definitely mollusks - oysters, and some clams and gastropods as stated in the other comments. The larger, very thick shells and triangular hinge structures are most likely something in the Ostreacea, probably Gryphaeidae. Some of the genera existed in the uppermost Triassic, but were much more common by the Cretaceous.