Mostly, yes! They use a very specific clay from a producer which went out of business a few years ago. They've bought up all the stock they can and use that clay for pretty much all of the character puppets, along with other materials. There was a great documentary on the BBC a couple of years back which went behind the scenes and talked about how they make and use the puppets.
> Feathers McGraw’s body, including his wicked flippers, is made of clay, crafted over a fully jointed internal metal armature. The red glove he famously wears as a remarkably effective disguise — one that somehow seems to fool Wallace every time — is made of silicone. His eyes are glass pins, while his beak is fast-cast resin. The tea-stained mug he grips is resin. He comes displayed on a wooden base.
"Curses, Gromit! Fire destroys animation firm's history on eve of box office triumph"
https://www.theguardian.com/uk/2005/oct/11/arts.media
That looks to be what's being auctioned.
Auction link: https://propstoreauction.com/auctions/catalog/id/549?page=1
https://www.youtube.com/watch?v=f19hF7-nT8g
/s
> Feathers McGraw’s body, including his wicked flippers, is made of clay, crafted over a fully jointed internal metal armature. The red glove he famously wears as a remarkably effective disguise — one that somehow seems to fool Wallace every time — is made of silicone. His eyes are glass pins, while his beak is fast-cast resin. The tea-stained mug he grips is resin. He comes displayed on a wooden base.