Thank goodness for signage, as I didn’t have a clue what this was, but apparently, it’s a polyhedral dial from the seventeenth century. It’s a sundial, which allows the user to see the time in a number of different ways. Initially I thought that it was wooden, but it’s made from stone and the figures are painted on. Only one of the gnomons remains (it’s on the rear side, so not visible in the photo), which are the little things which stick out to cast the shadow (I’m not sure that’s the most technical explanation).
The museum have a better photo at https://catalogue.museogalileo.it/gallery/PolyhedralDialInv2495.html.