From what I understand, these blocks were designed by Alfredo Barbini while at Cenedese. But as nothing in Murano is that clear-cut, I've seen very similar blocks with salviati labels. Fratelli Toso also did fishy things, but the fish lack the detail that you see in this block.
I think Cenedese is a very fair attribution. The piece is definitely mid-20th Century, as the quality is very high.