I’ve come across a couple of violins like this Storioni where the corner blocks had apparently been replaced. Signs of this are: none of the linings have been morticed into the blocks, and the linings ends have been cut on a line parallel with the line that bisects the rib miter allowing a new block to be slid into place.
I’m not really clear why you might want to replace corner blocks, generally you try to preserve as much original wood and structure as possible. I couldn’t see myself ever having to do this job but then I got a fiddle that was missing the top rib and corner block and so , with a little trepidation I set out to fit a new block from the inside. It turned out to be easier than I thought it would be.