They look like bean bags or the gel-filled analogs. You'll see them in museums for exactly the purpose shown. Gotta dissipate that weight.
Way back when I was in grad school and interning at the Oriental Institute, we would have to regularly shift certain objects that could be malleable thanks to gravity -- objects on display from the old days, especially gold or fabrics, would often distort if kept in place too long. Displays now have a lot of cushioning built in to them that they didn't used to. Like if you go see some of the Mycenaean Linear B tablets, they often sag some in the middle thanks to being displayed on an angle like a decorative plate for a century. Reminds me of when people wonder why the asphalt in their driveway dips where they always park their car even when nothing is contacting the ground at that spot; turns out gravity has something to say about a 500 pound engine block hanging over the same spot for years.